Fix determination of push type for empty notifications with badge 0

[cassidylaidlaw]

Without this change, it is impossible to send an empty notification that just sets the app badge to 0 (removes the badge). If notification.badge is 0, then APNsClient.send_notification_async will calculate the push type as background, which will not update the badge number. The solution is to compare notification.badge explicitly to None.

[Pr0Ger]

@cassidylaidlaw Thanks for contribution.

Btw, send_notification_async()/send_notification_batch() has param push_type to override this inferring. Dunno why it's not exposed in send_notification() method, I'll fix it in next release

[cassidylaidlaw]

Thanks for merging!