Fix two registration errors for unusual provider emails

Summary:
See <https://github.com/facebook/phabricator/issues/541>.

  - If a provider returns the email `""` or `"0"`, we currently don't let the user edit it and thus don't let them register.
  - If a provider returns an invalid email like `"!!!"` (permitted by GitHub, e.g.), we show them a nonsense error message.

Instead:

  - Pretend we didn't get an address if we get an invalid address.
  - Test the address strictly against `null`.

Test Plan: Registered on Phabricator with my GitHub email set to `""` (empty string) and `"!!!"` (bang bang bang).

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8528