Unfortunately, once you have a "mask" in place - the "Empty is NULL" option is not available to clear the column just by the user backspacing or Select / Deleting the data in the edit control. You would need to provide some PowerScript assistance.
In PB 12.6 you do have a new "Use Format" option on the DW column's that can make the EditMask disappear (hence your 00:00) - but you would have to upgrade your PB version.
Even though an EditMask is in vogue - I think that we should still have an "Empty is NULL" option.