Avoid panic with positional-only arguments in `PYI019` (#6350)
## Summary
Previously, failed on methods like:
```python
@classmethod
def bad_posonly_class_method(cls: type[_S], /) -> _S: ... # PYI019
```
Since we check if there are any positional-only or non-positional
arguments, but then do an unsafe access on `parameters.args`.
Closes https://github.com/astral-sh/ruff/issues/6349.
## Test Plan
`cargo test` (verified that `main` panics on the new fixtures)
