Fix GitHub OAuth Registration for users without a name

Summary:
Github allows you to have an account without a real name. The OAuth controller
actually handles this fine, mostly, except that it calls a bogus method. Also
there is some null vs empty string confusion.

Test Plan:
Deleted my name on Github and then registered for an account on Phabricator.

Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran
CC: anjali, aran, tuomaspelkonen
Differential Revision: 247