authorization services should throw an exception if resource being checked is null.
 - not a big deal as `RegistrationResource` and `ConferenceResource` obviously check the resources and return 404s
 - `RegistrantResource` though it's never possible to have a Registrant not associated with a Registration doesn't explicitly check that.
 - at the time of this commit, every `Registrant` was positively associated with a `Registration`
Merge branch 'master' of http://github.com/CruGlobal/conf-registration-api