Guard against `None` value of User.email attribute

If email was not specified during User creation (it's not required),
it will be `None`. It would be safer to guard against already existing
`None` values than to prevent creating new user with email == `None`.

Change-Id: Id81a94491918190ceae4b08cd3293250bb4ab194
Closes-Bug: #1383630
Merge "Guard against `None` value of User.email attribute"