This is by design since version 7.2.6 (Oct 2017). The empty value is stored as SQL null. If you must change, update in set_col_edittype() method in cls_datagrid.php.
Require source code - commercial licenses only:
In the last line of set_col_edittype(),
$keyvalue_pair = ($required ? '' : 'null') .":;".$select_list;
Change it to:
$keyvalue_pair = ":;".$select_list;