Don't fail on removing star labels from change without star labels

Setting no star labels on a change that doesn't have star labels should
be a no-op, but it currently fails with 500 Internal Server Error
because it attempts to delete a ref that doesn't exist.

Change-Id: Id180e4ff4f928aa229e29ed030b7af640e4605ff
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/StarredChangesUtil.java

@@ -476,6 +476,11 @@ public class StarredChangesUtil {
 
   private void deleteRef(Repository repo, String refName, ObjectId oldObjectId)
       throws IOException, OrmException {
+    if (ObjectId.zeroId().equals(oldObjectId)) {
+      // ref doesn't exist
+      return;
+    }
+
     RefUpdate u = repo.updateRef(refName);
     u.setForceUpdate(true);
     u.setExpectedOldObjectId(oldObjectId);