FirebaseMessagingAdminException: messaging/invalid-argument: Value type for APS key [alert] is a Dictionary or String.

[binemmanuel]

Describe the bug
After going through the code I understand that the _toProto() method serialises the ApnsConfig Object resulting in this exception as it creates nullable fields like;

{"alert": null, "sound": {"name": "default"}, ...}

To Reproduce

ApnsConfig(
    payload: ApnsPayload(
        aps: Aps(
            sound: CriticalSound(name: 'default'),
        ),
    ),
)

Expected behaviour
If the expected result was the below not including fields that weren't passed to the object then the said exception wouldn't be a thing.

{"sound": {"name": "default"}, ...}