[mlir] verify against nullptr payload in transform dialect

When establishing the correspondence between transform values and
payload operations or parameters, check that the latter are non-null and
report errors. This was previously allowed for exotic cases of partially
successfull transformations with "apply each" trait, but was dangerous.
The "apply each" implementation was reworked to remove the need for this
functionality, so this can now be hardned to avoid null pointer
dereferences.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D141142