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