Support Optional/Union types in script runner

[elliotgunton]

Pre-bug-report checklist

1. This bug can be reproduced using pure Argo YAML

• Yes 👉 Please report a bug to the Argo Workflows GitHub 👈
• No

If yes, it is more likely to be an Argo bug unrelated to Hera. Please double check before submitting an issue to Hera.

2. This bug occurs in Hera when...

• exporting to YAML
• submitting to Argo
• running on Argo with the Hera runner
• other:

Bug report

Describe the bug
A clear and concise description of what the bug is:

Using a function with an optional (string, but I think could be any type) argument in the runner results in the following stack trace:

Error log if applicable:

  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/runner.py", line 6, in <module>
    _run()
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/_runner/util.py", line 254, in _run
    result = _runner(args.entrypoint, kwargs_list)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/_runner/util.py", line 225, in _runner
    raise e
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/_runner/util.py", line 222, in _runner
    output = _save_annotated_return_outputs(function(**kwargs), output_annotations)
                                            ^^^^^^^^^^^^^^^^^^
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/_runner/util.py", line 58, in inner
    filtered_kwargs = {key: _parse(value, key, f) for key, value in kwargs.items() if _is_kwarg_of(key, f)}
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/_runner/util.py", line 58, in <dictcomp>
    filtered_kwargs = {key: _parse(value, key, f) for key, value in kwargs.items() if _is_kwarg_of(key, f)}
                            ^^^^^^^^^^^^^^^^^^^^^
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/_runner/util.py", line 89, in _parse
    if _is_str_kwarg_of(key, f) or _is_artifact_loaded(key, f) or _is_output_kwarg(key, f):
       ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/layers/.../requirements/lib/python3.11/site-packages/hera/workflows/_runner/util.py", line 142, in _is_str_kwarg_of
    return issubclass(type_, str)
           ^^^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 1 must be a class

To Reproduce
Full Hera code to reproduce the bug:

from hera.workflows import Workflow, script

@script(constructor="runner", image="my-image")
def optional_str(my_str: Optional[str], another_str: str | None):  # note the old/new ways to specify optional
    print(my_str)
    print(another_str)

with Workflow(name="my-workflow", entrypoint="optional-str") as w:
    optional_str()

Expected behavior
A clear and concise description of what you expected to happen:
No error trace.

Environment

• Hera Version: 5.14.3
• Python Version: 3.11
• Argo Version: 3.4.16

Additional context
Add any other context about the problem here.
We can currently work around this by using an empty string as the default.

@script(constructor="runner", image="my-image")
def optional_str(my_str: str = "", another_str: str = ""):
    print(my_str)
    print(another_str)