[mlir] teach expensive-checks transform mode about empty handle

The transform dialect interpreter features the expensive-checks mode
that acts as an embedded sanitizer to track use-after-consume of
transform handles. Its logic is based on the relations between payload
operations, which made it silently ignore empty handles that are
consumed. Also catch and report this case because the remaining code may
hit an assertion on attempting to access a consumed handle (that is
removed from the mapping).

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D151560