synchronized boolean isEmpty() {
      return missingStoreMessages == null || receivedStoreMessagesWithUpdatesPending == null || (
          missingStoreMessages.isEmpty() && receivedStoreMessagesWithUpdatesPending.isEmpty());
    }
Checks if this metadata response is empty. I.e, checks if there are no keys that are either missing in local store
or have updates pending.
@return true if both missingStoreMessages and receivedStoreMessagesWithUpdatesPending are empty.
ambry-replication/src/main/java/com/github/ambry/replication/ReplicaThread.java