42: Chametz isn’t Batel B’shishim on Pesach. Why would it be Batel before? We say Chozer V’neor. By your logic, why not say Bittul B’rov?
That aside, Chametz before Pesach is a D’var Heter, not Issur. There is a significant number of Achronim (spearheaded by the Chochmas Adam) who hold that the whole concept of Bittul is never Shayach by Heter.