Token in messaging:didReceiveRegistrationToken: should be nullable

[charlotteliang]

#5339 fixes an issue when token is deleted, the MessagingDelegate messaging:didReceiveRegistrationToken: should be triggered with a nil token returned. Hence the token can be nullable.
This is a breaking API that should be done in the next Firebase breaking change release.

[ryanwilson]

@chliangGoogle tagging so we don't miss this in Firebase 7