[mlir] simpler transform dialect silenceable failures

Simplify the handling of silenceable failures in the transform dialect.
Previously, the logic of `TransformEachOpTrait` required that
`applyToEach` returned a list of null pointers when a silenceable
failure was emitted. This was not done consistently and also crept into
ops without this trait although they did not require it. Handle this
case earlier in the interpreter and homogeneously associated preivously
unset transform dialect values (both handles and parameters) with empty
lists of the matching kind. Ignore the results of `applyToEach` for the
targets for which it produced a silenceable failure. As a result, one
never needs to set results to lists containing nulls. Furthermore, the
objects associated with transform dialect values must never be null.

Depends On D140980

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D141305