DND_Reborn
The High Aldwin
What is damage? Answer: a loss of hp. What does 1 damage mean. It means you lost 1 hp. What does 0 damage mean? It means you lost 0 hp, that your hp remained unchanged.
When a net is used on you then you lose 0 hp. The same with the sleep, when sleep is used on you then you lose 0 hp. Thus, Both sleep and the net deal 0 damage. The null set doesn’t apply to them because as just demonstrated, 0 damage has a meaning and both the net and sleep effects fall under that meaning.
f(x) = 0 is the mathematical model you are looking for to describe your interpretation of net damage. That is, no matter what input you use you always evaluate to 0. (This is in stark contrast to the null set)
HP is an abstract value lost when a character is injured by an attack, spell, etc. A net does not injure a player so it does not have an effect on HP.
You are blatantly incorrect in this case and not demonstrating anything convincing. There are plenty of features and such in D&D that have no damaging effect. Sleep, Grappling, etc. Consider the Tendril attack of a Roper. It is an Melee Weapon Attack (+7, reach 50 ft., one creature). On a hit, the target is grappled (no damage dealt). A Roper has a STR +4 modifier, but despite this being a melee weapon attack, the damage isn't 4 or anything else. There is no damaging effect, it just isn't there. The Roper still gets the STR +4 modifier on the attack roll (proficiency bonus 3 +4 = 7 attack modifier) but doesn't deal damage on the attack. It isn't that it deals 0 damage, but that it doesn't deal damage. Important distinction you seem to be missing IMO.
So the null set is precisely what you are dealing with. There is no damage dealt. We are not dealing with the function f(x) = 0.
All that being the case, if you are your table's DM, interpret as 0. No worries. If you are player and want it that way, discuss it with your DM. In my first post in this thread I argued for modifying damage and rationale as to how it could be justified if you wanted to play it that way. But in a pure interpretation of the RAW and RAI, that isn't how nets work.
In a later post I posed the question: you add Dex mod to the attack roll with a net, do you add Dex mod to the damage? If you are playing that way, then yes, modify the damage and treat it as 0 if you want. If you have been adding Dex mod to the attack roll, but not having the net "damage" adjusted by Dex mod, then you shouldn't modify it through other means.