Presumably, if a=b is strict then effectively nothing could match as the strict qual ensures NULLs never match and the IS NULL only allows NULLs.
Yeah, in this case the two restrictions are self-inconsistent and that makes the relation dummy and need not be scanned. I think the planner would figure that out in relation_excluded_by_constraints when the GUC constraint_exclusion is on, but in a different way than ECs.
