Fix NPE in REST /api/v1/group/{groupName}/@users

Using LDAP as an user directory and PostgreSQL as a group directory

1. Add a user in LDAP pertaining to group X
2. Suppress the user from the LDAP source (Nuxeo does suppress the user neither from the group nor from the metadata, and this is fine as such)
3. Request REST /api/v1/group/{groupName}/@users and you will get the exception:

   2018-01-26 08:18:12,231 ERROR [http-bio-0.0.0.0-8080-exec-3] [org.nuxeo.ecm.platform.web.common.exceptionhandling.DefaultNuxeoExceptionHandler] java.lang.NullPointerException
   	at org.nuxeo.ecm.platform.usermanager.io.NuxeoPrincipalJsonWriter.writeEntityBody(NuxeoPrincipalJsonWriter.java:115)
   	at org.nuxeo.ecm.platform.usermanager.io.NuxeoPrincipalJsonWriter.writeEntityBody(NuxeoPrincipalJsonWriter.java:98)
   	at org.nuxeo.ecm.core.io.marshallers.json.ExtensibleEntityJsonWriter.write(ExtensibleEntityJsonWriter.java:79)
   	at org.nuxeo.ecm.core.io.marshallers.json.AbstractJsonWriter.write(AbstractJsonWriter.java:76)
   	at org.nuxeo.ecm.core.io.marshallers.json.DefaultListJsonWriter.write(DefaultListJsonWriter.java:124)
   	at org.nuxeo.ecm.core.io.marshallers.json.DefaultListJsonWriter.write(DefaultListJsonWriter.java:70)
   	at org.nuxeo.ecm.core.io.marshallers.json.AbstractJsonWriter.write(AbstractJsonWriter.java:76)
   	at org.nuxeo.ecm.webengine.jaxrs.coreiodelegate.PartialCoreIODelegate.writeTo(PartialCoreIODelegate.java:121)
   	at com.sun.jersey.spi.container.ContainerResponse.write(ContainerResponse.java:302)
   	at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1510)
   	at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1419)
   	at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1409)
   	at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:409)
   	at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:558)
   	at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:733)
   	at org.nuxeo.ecm.webengine.app.jersey.WebEngineServlet.containerService(WebEngineServlet.java:178)
   	at org.nuxeo.ecm.webengine.app.jersey.WebEngineServlet.service(WebEngineServlet.java:155)
   	at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.elasticsearch.ElasticSearchFilter.doFilter(ElasticSearchFilter.java:55)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.webengine.jaxrs.session.SessionCleanupFilter.run(SessionCleanupFilter.java:50)
   	at org.nuxeo.ecm.webengine.jaxrs.HttpFilter.doFilter(HttpFilter.java:49)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.webengine.app.WebEngineFilter.doFilter(WebEngineFilter.java:92)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.webengine.jaxrs.context.RequestContextFilter.run(RequestContextFilter.java:48)
   	at org.nuxeo.ecm.webengine.jaxrs.HttpFilter.doFilter(HttpFilter.java:49)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.core.management.jtajca.internal.Log4jWebFilter.doFilter(Log4jWebFilter.java:69)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.ui.web.auth.NuxeoAuthenticationFilter.doFilterInternal(NuxeoAuthenticationFilter.java:608)
   	at org.nuxeo.ecm.platform.ui.web.auth.service.NuxeoAuthFilterChain.doFilter(NuxeoAuthFilterChain.java:51)
   	at org.nuxeo.ecm.platform.ui.web.auth.oauth.NuxeoOAuthFilter.doFilter(NuxeoOAuthFilter.java:122)
   	at org.nuxeo.ecm.platform.ui.web.auth.service.NuxeoAuthFilterChain.doFilter(NuxeoAuthFilterChain.java:49)
   	at org.nuxeo.ecm.platform.ui.web.auth.oauth2.NuxeoOAuth2Filter.doFilter(NuxeoOAuth2Filter.java:82)
   	at org.nuxeo.ecm.platform.ui.web.auth.service.NuxeoAuthFilterChain.doFilter(NuxeoAuthFilterChain.java:49)
   	at org.nuxeo.ecm.platform.ui.web.auth.NuxeoAuthenticationFilter.doFilter(NuxeoAuthenticationFilter.java:411)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.web.common.exceptionhandling.NuxeoExceptionFilter.doFilter(NuxeoExceptionFilter.java:77)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.web.common.encoding.NuxeoEncodingFilter.doFilter(NuxeoEncodingFilter.java:75)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:747)
   	at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:485)
   	at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:410)
   	at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:337)
   	at org.nuxeo.ecm.restapi.server.APIServlet.service(APIServlet.java:50)
   	at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.elasticsearch.ElasticSearchFilter.doFilter(ElasticSearchFilter.java:55)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.web.common.requestcontroller.filter.NuxeoThreadTrackerFilter.doFilter(NuxeoThreadTrackerFilter.java:43)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.core.management.jtajca.internal.Log4jWebFilter.doFilter(Log4jWebFilter.java:69)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.ui.web.rest.FancyURLFilter.doFilter(FancyURLFilter.java:120)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.web.common.requestcontroller.filter.NuxeoRequestControllerFilter.doFilter(NuxeoRequestControllerFilter.java:146)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.webdav.service.WIRequestFilter.doFilter(WIRequestFilter.java:61)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at com.thetransactioncompany.cors.CORSFilter.doFilter(CORSFilter.java:197)
   	at com.thetransactioncompany.cors.CORSFilter.doFilter(CORSFilter.java:274)
   	at org.nuxeo.ecm.platform.web.common.requestcontroller.filter.NuxeoCorsFilter.doFilter(NuxeoCorsFilter.java:49)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.web.common.exceptionhandling.NuxeoExceptionFilter.doFilter(NuxeoExceptionFilter.java:77)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.nuxeo.ecm.platform.web.common.encoding.NuxeoEncodingFilter.doFilter(NuxeoEncodingFilter.java:75)
   	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
   	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
   	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
   	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
   	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
   	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
   	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:436)
   	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078)
   	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:625)
   	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
   	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
   	at java.lang.Thread.run(Thread.java:745)