Empty extra in list_items

If you have a custom field that's based on a list and the default value is set to a list item where the extra field is empty ('') instead of NULL, you cannot set the default value back to 'Select a Default'. You CAN set it to other default values

Extra is set to empty if you erase the abbreviation and save it. Should save as NULL instead of empty

Note: for some reason this didn't work

if ($k == 'abbrev' && empty($vars[$k]))
                $this->set($v, NULL);