[flang] Fix passing NULL to OPTIONAL procedure pointers (#80267)

Procedure pointer lowering used `prepareUserCallActualArgument` because
it was convenient, but this helper was not meant for POINTERs when
originally written and it did not handled passing NULL to an OPTIONAL
procedure pointer correctly.

The resulting argument should be a disassociated pointer, not an absent
pointer (Fortran 15.5.2.12 point 1.).

Move the logic for procedure pointer argument "cooking" in its own
helper to avoid triggering the logic that created an absent argument in
this case.