collision_mask also have a mandatory layers field. I'm not familiar enough with factorio modding to understand what I need to do with that information, so I just removed the collision_mask from the item
I think it's used for "invisible" constant combinator which actually produces signals for output wires, which is set to empty value there because it's a kind of "virtual" entity, only there to work around inability to set signals directly on the wires or connection points (not sure if it's still a limitation btw - mod is pretty old).
These "virtual" combinators should normally always be removed along with the visible MLC part, so I think not removing collision_mask should probably be fine.
But that mask might be larger than arithmetic combinator which this is placed on top of, so might make collisions with it a little strange.
Though given that both entities are small, probably won't be noticeable, but ideally you'd still want to make this "virtual" combinator non-collidable, which might be as simple as setting that mandatory "layers" prop to an empty table or something like that.