Fix exception in OAuthServerAuthController

Summary:
We've been building a Jenkins plugin that allows you to use your Phabricator login details in Jenkins using the inbuilt OAuthServer. I noticed that when making a request to /oauthserver/auth/?client_id=&response_type=code I get an error back from the server.

I've traced this down to two bugs in PhabricatorOAuthServerAuthController, the first causes a null value error on $access_token_uri, and the second fails on userHasAuthorizedClient without a $scope array.

Test Plan: Go to /oauthserver/auth/?client_id=<client_id>&response_type=code and get a valid authorization code back

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4808