Only register RollbarServiceProvider when token is set
Only register RollbarServiceProvider when token is set

This enhances post-install experience by fixing an Exception thrown
when no access token for Rollbar is provided.

timegridio/timegrid,alariva/timegrid,timegridio/timegrid,timegridio/timegrid,alariva/timegrid,alariva/timegrid,alariva/timegrid