[flang][hlfir][NFC] catch dynamically optional args with no lowering

These should be lowered with genOptionalValue as in D154897, but I
haven't found any cases where this code path is actually hit (flang
tests, gfortran test suite), so I don't think it would be testable.
Adding an assertion for if this code path ever becomes live.

Differential Revision: https://reviews.llvm.org/D155477