Add process crashes if it cannot find a shell

[wk9874]

Description of Bug

When running inside an apptainer on HPC via slurm, I get this error:

  File ".venv/lib64/python3.12/site-packages/simvue/run.py", line 998, in add_process
    self._executor.add_process(
  File ".venv/lib64/python3.12/site-packages/simvue/executor.py", line 311, in add_process
    command += self._kwarg_assembly(cmd_kwargs, executable=executable)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib64/python3.12/site-packages/simvue/executor.py", line 158, in _kwarg_assembly
    _shell_is_pwsh: bool = any(
                           ^^^^
  File ".venv/lib64/python3.12/site-packages/simvue/executor.py", line 159, in <genexpr>
    shell in get_current_shell() for shell in ("pwsh", "powershell")
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: argument of type 'NoneType' is not iterable

This is because get_current_shell returns str | None, but the None case is not handled inside the executor