AeroDoc: Avoid registration with nil value for device token

In AeroDoc we try to stash the token:
https://github.com/aerogear/aerogear-push-quickstart-ios/blob/master/ProDoctor/ProDoctor/AGAppDelegate.m#L49

However, when APNs registration fails, this callback is invoked:
https://github.com/aerogear/aerogear-push-quickstart-ios/blob/master/ProDoctor/ProDoctor/AGAppDelegate.m#L53

That means there is no stashed token.

Now after we got a successful login, against the AeroDoc backend, we should check if the token is nil or not, before we "blindly" perform a registration with the UnifiedPush Server:
https://github.com/aerogear/aerogear-push-quickstart-ios/blob/master/ProDoctor/ProDoctor/Classes/Controllers/AGLoginViewController.m#L138

=> A 'nil' check should be done before execution of that method