> Writing a check for $0 is not an instruction to pay.
By describing it as a check you've already acknowledged it as an instruction to pay.
> Paying $0 is equivalent to not paying.
It's equivalent in terms of the final balance, but it's not the same thing in other respects. If I sent you a check for $0, you would wonder how I got your address. There are a variety of ways that it would be different from me not sending you a check for any amount of money at all.