Wrong condition for some messages triggers php warning

Added patch

I've found that depending on the "to" parameter, Firebase returns a different response.

According to the documentation at https://firebase.google.com/docs/cloud-messaging/send-message, if the notification is sent using an app register token, the response will be:

// Success
{ 
  "multicast_id": 108,
  "success": 1,
  "failure": 0,
  "canonical_ids": 0,
  "results": [
    { "message_id": "1:08" }
  ]
}

But if the notification is sent to a topic, the response will be:

//Success example:
{
  "message_id": "1023456"
}

//failure example:
{
  "error": "TopicsMessageRateExceeded"
}

Then, here's a new version of the patch to check for notification failures.

Uploaded the patch with a wrong name. Here is the correct one