Add null checks for auth plugin processing

[Jotschi]

Running auth plugins which return results which may contain null fields can cause OLocalHashTableException: Null keys are not supported.. Role names and group names must be checked for null. A warning should be printed when a null values was used.
Lookup should be done via uuid or name.

cms-api-getmesh-master-0 getmesh Exception `562B569E` in storage `plocal:/graphdb/storage`: 3.0.28 - Veloce (build 7f2fc09560546cde8f7cfacd91423de88b192678, branch UNKNOWN)
cms-api-getmesh-master-0 getmesh com.orientechnologies.orient.core.exception.OLocalHashTableException: Null keys are not supported.
cms-api-getmesh-master-0 getmesh DB name="storage"
cms-api-getmesh-master-0 getmesh Component Name="MeshVertexImpl"
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.storage.index.hashindex.local.OLocalHashTable.checkNullSupport(OLocalHashTable.java:1336)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.storage.index.hashindex.local.OLocalHashTable.get(OLocalHashTable.java:181)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.storage.index.engine.OHashTableIndexEngine.get(OHashTableIndexEngine.java:159)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.storage.impl.local.OAbstractPaginatedStorage.doGetIndexValue(OAbstractPaginatedStorage.java:2979)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.storage.impl.local.OAbstractPaginatedStorage.getIndexValue(OAbstractPaginatedStorage.java:2960)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.index.OIndexOneValue.get(OIndexOneValue.java:58)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.index.OIndexOneValue.get(OIndexOneValue.java:45)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.index.OIndexAbstractDelegate.get(OIndexAbstractDelegate.java:59)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.index.OIndexTxAwareOneValue.get(OIndexTxAwareOneValue.java:238)
cms-api-getmesh-master-0 getmesh at com.orientechnologies.orient.core.index.OIndexTxAwareOneValue.get(OIndexTxAwareOneValue.java:38)
cms-api-getmesh-master-0 getmesh at com.gentics.mesh.graphdb.index.OrientDBIndexHandler.findByUuid(OrientDBIndexHandler.java:406)
cms-api-getmesh-master-0 getmesh at com.gentics.mesh.core.data.root.RootVertex.findByUuid(RootVertex.java:170)
cms-api-getmesh-master-0 getmesh at com.gentics.mesh.auth.MeshOAuth2ServiceImpl.runPlugins(MeshOAuth2ServiceImpl.java:378)
cms-api-getmesh-master-0 getmesh at com.gentics.mesh.auth.MeshOAuth2ServiceImpl.lambda$syncUser$7(MeshOAuth2ServiceImpl.java:227)
cms-api-getmesh-master-0 getmesh at com.gentics.mesh.graphdb.OrientDBDatabase.tx(OrientDBDatabase.java:417)
cms-api-getmesh-master-0 getmesh at com.gentics.mesh.graphdb.spi.Database.lambda$maybeTx$6(Database.java:189)
cms-api-getmesh-master-0 getmesh at io.vertx.reactivex.core.Vertx$2.handle(Vertx.java:597)
cms-api-getmesh-master-0 getmesh at io.vertx.reactivex.core.Vertx$2.handle(Vertx.java:595)
cms-api-getmesh-master-0 getmesh at io.vertx.core.impl.ContextImpl.lambda$executeBlocking$2(ContextImpl.java:316)
cms-api-getmesh-master-0 getmesh at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
cms-api-getmesh-master-0 getmesh at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
cms-api-getmesh-master-0 getmesh at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
cms-api-getmesh-master-0 getmesh at java.base/java.lang.Thread.run(Unknown Source)
cms-api-getmesh-master-0 getmesh 13:54:33.463 [] ERROR [vert.x-worker-thread-4] - Error while executing mapping plugin {auth-plugin}. Ignoring result.
cms-api-getmesh-master-0 getmesh com.orientechnologies.orient.core.exception.OLocalHashTableException: Null keys are not supported.