You could return if in that new if statement.

As you noted, the None argument is ignored there, that is because
_tkinter checks for a None parameter, and if it happens to be a None, it
then stops processing new arguments, so this is not directly related to
the delete command of the Menu widget.

Regarding a test suite.. it would be very nice to get one for Tkinter.
Some weeks ago I did some googling but found no attempts for my surprise.