Fixes an index error that is thrown in the django admin.
Fixes an index error that is thrown in the django admin.

On Django 1.9 if the gcm device id is null, an error is thrown
on the to_python method of the HexIntegerField class.

matthewh/django-push-notifications,GaleDragon/django-push-notifications,cristiano2lopes/django-push-notifications,gkirkpatrick/django-push-notifications,jamaalscarlett/django-push-notifications,rsalmaso/django-push-notifications,Adys/django-push-notifications,hylje/django-push-notifications,shigmas/django-push-notifications,Tictrac/django-push-notifications,nnseva/django-push-notifications,lukeburden/django-push-notifications,CustomerSupport/django-push-notifications,1vank1n/django-push-notifications,jleclanche/django-push-notifications,fsto/django-push-notifications