NullPointerException when using cloud messaging

* Library version: 9.4.3
* Firebase Product: cloud messaging

When we are sending messages in bulk to devices using device specific tokens, we randomly get the error below:

```
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at com.google.common.util.concurrent.AbstractFuture.getDoneValue(AbstractFuture.java:595)
at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:574)
at com.google.common.util.concurrent.FluentFuture$TrustedFuture.get(FluentFuture.java:91)
at com.google.common.util.concurrent.ForwardingFuture.get(ForwardingFuture.java:67)
at businessLayer.Messaging.sendPush(Messaging.java:777)
...
```

We are using the method sendEachAsync:
`BatchResponse firebaseResponse = FirebaseMessaging.getInstance(firebaseApp).sendEachAsync(firebaseMessages, pushDryRun).get();`

Besides the error showing up randomly when we are sending the messages, it seems that in older version of the library (<= 9.3.0), that the error is:

```
java.util.concurrent.ExecutionException: com.google.firebase.messaging.FirebaseMessagingException: com.google.firebase.messaging.FirebaseMessaging
at com.google.common.util.concurrent.AbstractFuture.getDoneValue(AbstractFuture.java:594)
at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:573)
at com.google.common.util.concurrent.FluentFuture$TrustedFuture.get(FluentFuture.java:91)
at com.google.common.util.concurrent.ForwardingFuture.get(ForwardingFuture.java:67)
at businessLayer.Messaging.sendPush(Messaging.java:777)
...
```

The problem for us is that it seems that this error started showing up randomly last month.
At least some messages seem to be sent, because we have info that users are seeing and clicking on the notifications.
So, it seems that we are correctly calling the procedure.
Any help would be welcomed.

Start at the sendEachAsync call and businessLayer.Messaging.sendPush in Messaging.java:777, then reproduce a bulk send to device-specific tokens on library version 9.4.3. Trace why the asynchronous result becomes a NullPointerException instead of the older FirebaseMessagingException, and verify that bulk sends report an actionable error while successful notifications remain unaffected.

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