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.