Address a bug where NullTime is still valid when data is 0000-00-00 00:00:00 in db

When insert an invalid date time value to a datetime field, MySQL will store in db as "0000-00-00 00:00:00".
So, when read "0000-00-00 00:00:00" value from db and convert to NullTime object, this object should be invalid instead of valid.