R1:f7b1ed722164

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…