Is there a GUI that allows null-moves as legal moves? Of course for engines the null-move heuristic already is an established technique.
Fritz allows null move entry via CTL+ALT+0. There may also be a menu option to do this.
Scid vs PC also allows this by capturing one king with the other or by using the keyboard to enter "--" i.e. hit the minus key twice.
Hiarcs also uses king takes king to enter a null move. There is also a menu option "Insert/Null Move" in the "Edit" menu, plus a keyboard shortcut of CTL+0
The notion of null-move is implicit to orthodox chess problem theory, and all specialist chess problem software will support it.
Firstly there is the notion of threat. After White plays a move in a conflict format (direct or self mate) the first consideration if there is no check is what would happen if White moved again immediately. Then the Black moves which would avert the threat are the only defences which need to be considered.
The second notion applies to helpmates as well. This is set play, where it is assumed that the first player misses their first move. Often the play contrasts the mainline play thematically.