send_onesignal_notification checkbox does not correctly display when onesignal_send_notification set to false

[kitwestneat]

Description:
The send_onesignal_notification checkbox is always checked on the edit post page if the global send notification is enable and the post page is enabled.

Steps to Reproduce Issue:

1. ensure global notification_on_post config is set true
2. create a new post
3. uncheck send_onesignal_notification checkbox
4. click save draft
5. observe checkbox is re-checked

Anything else:
It seems that get_post_meta returns '' for both false and null, making it impossible to tell if the user has explicitly set onesignal_send_notification to false. Does it make sense to change the meta valuse to 0 and 1 or something like that?