[flang] Fix bug converting actual argument to Fortran

This fixes a problem with converting the ubound call in the example
below back to Fortran (in this case, for writing to the .mod file).
One of the ActualArguments encountered in ProcedureRef::AsFortran is
a `std::nullopt`, so we need to handle that case.

```
module m
  real :: x(10)
  real :: y(ubound(x, dim=1))
end module
```

Original-commit: flang-compiler/f18@c5ace6b8241cd1ef86f0649941bf38386d99bdc4
Reviewed-on: https://github.com/flang-compiler/f18/pull/240