How to check if firebase query is empty

I want to delete the data from Firebase before unauthorizing. The problem is that mFirebaseRef.unauth() works only if query is not empty. But I need it to work even if query is empty.

final Firebase pushNotificationRef = new Firebase(Constant.FIREBASE_URL_PUSHNOTIFICATIONS);
    final Query queryRef = pushNotificationRef.orderByChild("deviceToken").equalTo(token);
    queryRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            if (dataSnapshot.exists()) {
                Log.e("MyTag", dataSnapshot.getKey());
                pushNotificationRef.child(dataSnapshot.getKey()).removeValue();
            }
            mFirebaseRef.unauth();
        }