[flang] Fix lowering of optional char proc args

Optional character function arguments were not being lowered
properly. As they are passed as a tuple, containing the (boxed)
function address and the character length, it is not possible for
fir.absent to handle it directly. Instead, a tuple needs to be
created and filled with an absent function address and a dummy
character length.

Fixes #60225

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D144743