Optional positional arguments throw an error in argparse

[joshlk]

Describe the bug
Optional positional arguments throw a "TypeError: 'required' is an invalid argument for positional" error in argparse.

To Reproduce

from dataclasses import dataclass
import simple_parsing

@dataclass
class Foo:
  a: int | None = simple_parsing.field(positional=True, default=None)

simple_parsing.parse(Foo)

Produces this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/python3.12/site-packages/simple_parsing/parsing.py", line 1044, in parse
    parsed_args = parser.parse_args(args)
                  ^^^^^^^^^^^^^^^^^^^^^^^
  File "/python3.12/argparse.py", line 1895, in parse_args
    args, argv = self.parse_known_args(args, namespace)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/python3.12/site-packages/simple_parsing/parsing.py", line 345, in parse_known_args
    self._preprocessing(args=args, namespace=namespace)
  File "/python3.12/site-packages/simple_parsing/parsing.py", line 549, in _preprocessing
    wrapped_dataclass.add_arguments(parser=self)
  File "/python3.12/site-packages/simple_parsing/wrappers/dataclass_wrapper.py", line 214, in add_arguments
    _ = group.add_argument(*wrapped_field.option_strings, **arg_options)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/python3.12/argparse.py", line 1459, in add_argument
    kwargs = self._get_positional_kwargs(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/python3.12/argparse.py", line 1575, in _get_positional_kwargs
    raise TypeError(msg)
TypeError: 'required' is an invalid argument for positional

Issue resolved in PR: #345