Optional Field args don't work as expected

[mure]

Describe the bug
When using the Field annotation on an arg to be converted into a JSON field, nullable types don't work as expected. Using Optional directly throws an error, but you can work around this by using the x | None syntax instead. However, this is causing the request body to include unintended nulls.

To Reproduce

@json
@args(Field("a"))
@patch("route")
def patch(self, a: Optional[str] = None):
   pass

Result:
TypeError: issubclass() arg 1 must be a class

@json
@args(Field("a"))
@patch("route")
def patch(self, a: str | None = None):
   pass

client.patch()

Result

{ "a": null }

It would be nice if this had the same behavior as Query params.