Fix NullReferenceException when push notification handler hasn't been registered.
Fix NullReferenceException when push notification handler hasn't been registered.

When a push was received without a handler, we would previously crash. This doesn't exist as an issue on other platforms, as we use other notification delivery systems there.

Fixes #27.

wujun4code/Parse-SDK-dotNET,AmeliaMesdag/Parse-SDK-dotNET,ParsePlatform/Parse-SDK-dotNET,ParsePlatform/Parse-SDK-dotNET,wujun4code/Parse-SDK-dotNET,AmeliaMesdag/Parse-SDK-dotNET