Fix FCM second retry sends

[Fizzadar]

When the first attempt succeeds for all but one pushkey the second attempt will end up populating both the to and registration_ids fields, which is rejected by FCM (400). For context, full errors looked like:

"[...] 400 from server, we have sent something invalid! Error: 'Must use either \"registration_ids\" field or \"to\", not both\\n'"