[mlir] reallow null results in TransformEachOpTrait

Previous changes in 98acd746 were overly
eager to disallow null payload everywhere. The semantics of
TransformEachOpTrait allows individual applications to return null
payloads as means of filtering out the operations to which they are not
applicable without emitting even a silenceable failure. This is a
questionable choice, but one apparently relied upon. Null payloads are
not supposed to leak outside of the trait.

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D143904