Fix HTTP 500 when running an unauthenticated request against REST API with Keycloak auth

Steps to reproduce:

1. Start keycloak

   docker run --name test-keycloak19.0.3 -d -p 8087:8080 -e KEYCLOAK_ADMIN=admin -e KEYCLOAK_ADMIN_PASSWORD=admin quay.io/keycloak/keycloak:19.0.3 start-dev 

2. import the attached realm nuxeo-realm.json (not really required)
3. Install nuxeo-keycloak addon and configure the nuxeo.conf properties

   nuxeo.keycloak.realm=nuxeorealm
   nuxeo.keycloak.resource=nuxeo
   nuxeo.keycloak.sslRequired=none
   nuxeo.keycloak.publicClient=false
   nuxeo.keycloak.confidentialPort=0
   nuxeo.keycloak.authServerUrl=http://localhost:8087
   nuxeo.keycloak.credentials.secret=eGEPEQNIH5AYUn5DflXoPMaNj4v1lotl
   

4. Start Nuxeo
5. Run this command

   curl -v -s -X POST "http://localhost:8080/nuxeo/site/automation/Auth.LoginAs" -d '{"params":{},"context":{}}' -H 'Content-Type: application/json'--compressed --insecure

6. Observe the response with HTTP 500
7. Observe also the error in the logs

   ERROR [WebEngineExceptionMapper] java.lang.IllegalStateException: Not authenticated user is trying to get a core session
   java.lang.IllegalStateException: Not authenticated user is trying to get a core session
   	at org.nuxeo.ecm.webengine.jaxrs.session.CoreSessionProvider.createSession(CoreSessionProvider.java:66) ~[nuxeo-webengine-jaxrs-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.jaxrs.session.CoreSessionProvider.getSessionRef(CoreSessionProvider.java:54) ~[nuxeo-webengine-jaxrs-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.jaxrs.session.CoreSessionProvider.getSession(CoreSessionProvider.java:61) ~[nuxeo-webengine-jaxrs-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.jaxrs.session.SessionFactory.getSession(SessionFactory.java:109) ~[nuxeo-webengine-jaxrs-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.jaxrs.session.SessionFactory.getSession(SessionFactory.java:105) ~[nuxeo-webengine-jaxrs-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.jaxrs.session.SessionFactory.getSession(SessionFactory.java:92) ~[nuxeo-webengine-jaxrs-2023.1.20.jar:?]
   	at org.nuxeo.ecm.automation.server.jaxrs.CoreSessionProvider.getValue(CoreSessionProvider.java:39) ~[nuxeo-automation-server-2023.0.159.jar:?]
   	at org.nuxeo.ecm.automation.server.jaxrs.CoreSessionProvider.getValue(CoreSessionProvider.java:35) ~[nuxeo-automation-server-2023.0.159.jar:?]
   	at com.sun.jersey.server.impl.inject.AbstractHttpContextInjectable$1.getValue(AbstractHttpContextInjectable.java:104) ~[jersey-server-1.19.4.jar:1.19.4]
   	at com.sun.jersey.server.spi.component.ResourceComponentInjector.inject(ResourceComponentInjector.java:222) ~[jersey-server-1.19.4.jar:1.19.4]
   	at com.sun.jersey.server.spi.component.ResourceComponentConstructor.construct(ResourceComponentConstructor.java:234) ~[jersey-server-1.19.4.jar:1.19.4]
   	at org.nuxeo.ecm.webengine.model.impl.AbstractResourceType.newInstance(AbstractResourceType.java:132) ~[nuxeo-webengine-core-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.model.impl.AbstractWebContext.newObject(AbstractWebContext.java:332) ~[nuxeo-webengine-core-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.model.impl.AbstractWebContext.newObject(AbstractWebContext.java:327) ~[nuxeo-webengine-core-2023.1.20.jar:?]
   	at org.nuxeo.ecm.webengine.model.impl.AbstractResource.newObject(AbstractResource.java:262) ~[nuxeo-webengine-core-2023.1.20.jar:?]
   	at org.nuxeo.ecm.automation.server.jaxrs.AutomationResource.getExecutable(AutomationResource.java:144) ~[nuxeo-automation-server-2023.0.159.jar:?]
   

    

Expected behavior: the unauthenticated request should request a HTTP 401