fix: empty birthday element submitted always '0000-00-00', change: empty is null defaulted to 1 (yes)

Due to some of the latest fixes the bd element lost its ability to save empty data as NULL. 
empty_is_null was one of my enhancements since 1.5 but really don't recall why I preferred to default it to 0. To be honest - NULL instead of '0000-00-00' seems to be much more usual need :)