title: Set date to NULL in MySQL database when leaving date field empty
body: <p>hi have a HTML 5 date field for release date but not all items has a release date so I want to be able  to set it to null and hide releasedate entirely for those items. When I leave the date field empty nothing happens. The previous date (2011-01-01) is still there (I thought they had a release date but i didn't know what it was so I just picked one sinced the table didn't allow null before). I have changed it to allow null now and I have managed to make it null in phpMyAdmin but not by using my own forms and php.</p>
  
  <p>Any ideas?</p>