If you send a data only push, and empty message is sent

[AndyGaskell]

There is an issue in Push/Notification.php whereby if you want to send a data only message, so, you leave the title, body etc fields blank, it'll still send a non-data push message, but just with blank content.

I've a code fix for this, so will do a PR this afternoon.

[Philipp91]

I first believed I was affected by this, but then I stopped using the Notification class and started using Push instead, which works as intended, at least in some initial testing. So maybe this can be closed.