Fix explorer error on override

2020-10-12T09:14:56,075 ERROR [http-nio-127.0.10.1-8080-exec-4] [org.nuxeo.apidoc.browse.Distribution] java.lang.NullPointerException
java.lang.NullPointerException: null
	at org.nuxeo.ecm.core.query.sql.NXQL.escapeStringInner(NXQL.java:268) ~[nuxeo-core-query-11.3.56.jar:?]
	at org.nuxeo.ecm.core.query.sql.NXQL.escapeString(NXQL.java:252) ~[nuxeo-core-query-11.3.56.jar:?]
	at org.nuxeo.apidoc.api.QueryHelper.select(QueryHelper.java:62) ~[nuxeo-apidoc-core-20.0.0.jar:?]
	at org.nuxeo.apidoc.repository.RepositoryDistributionSnapshot.getChild(RepositoryDistributionSnapshot.java:170) ~[nuxeo-apidoc-core-20.0.0.jar:?]
	at org.nuxeo.apidoc.repository.RepositoryDistributionSnapshot.getContribution(RepositoryDistributionSnapshot.java:249) ~[nuxeo-apidoc-core-20.0.0.jar:?]
	at org.nuxeo.apidoc.browse.ComponentWO.override(ComponentWO.java:57) ~[nuxeo-apidoc-webengine-20.0.0.jar:?]
	at jdk.internal.reflect.GeneratedMethodAccessor167.invoke(Unknown Source) ~[?:?]
	at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
	at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?]
	at com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60) ~[jersey-server-1.19.4.jar:1.19.4]
	at com.sun.jersey.server.impl.model.method.dispatch.AbstractResourceMethodDispatchProvider$ObjectOutInvoker._dispatch(AbstractResourceMethodDispatchProvider.java:258) ~[jersey-server-1.19.4.jar:1.19.4]
	at com.sun.jersey.server.impl.model.method.dispatch.ResourceJavaMethodDispatcher.dispatch(ResourceJavaMethodDispatcher.java:75) ~[jersey-server-1.19.4.jar:1.19.4]