If you want to make null a variant that is fine with me and even a shortcut Null candid keyword as long as over the wire it is a variant. The Option-type can as well be a variant over the wire I think that is better
We already went over this, reserved can be used here (or empty).
You can use reserved here as well. (And side point why on earth would anyone want to use a canister that is made specific for a map key,value store as a set? What an example.)
- This can only be backwards compatible if the canister checks for people sending options and skips them.
- a canister which makes new clients send it a null value is just a waste.
- if this was a legitimate case, the canister can use the type: Reserved .