Correctly handle missing refs/meta/config branch

If the refs/meta/config branch does not exist, getRevision() used to
throw NPE when trying to access the ProjectDetail. This is because
on a missing branch the revision field is null, and must return a
null to the caller rather than copying the value.

Change-Id: If4dd771d39382b89443a234f16d13a7a4e6838c8

gerrit-server/src/main/java/com/google/gerrit/server/git/VersionedMetaData.java

@@ -66,7 +66,7 @@ public abstract class VersionedMetaData {
 
   /** @return revision of the metadata that was loaded. */
   public ObjectId getRevision() {
-    return revision.copy();
+    return revision != null ? revision.copy() : null;
   }
 
   /** Initialize in-memory as though the repository branch doesn't exist. */