(CE-769) Fix fatal in LookupUser

A zero date (0000-00-00) in PHP on 64-bit systems results in a negative
date with strtotime instead of false like it used to. Since this isn't
a valid date and is equivalent to null when stored in the DB for the
birthdate, ignore it.
(CE-769) Don't try to convert the birthdate into a timestamp

The user_birthdate field is a date field, but in these two methods
we were using timestampOrNull which will return false when not given
a valid timestamp. So, when a date was passed to it instead of a
timestamp, false would go here, which would result in a user_burthdate
of 0000-00-00. So, by removing the timestampOrNull call, we match the
way we handle this in ExternalUser_Wikia.php which is what will be used
most of the time.
Merge pull request #3462 from Wikia/CE-769

(CE-769) Fix user birthdate error on LookupUser