`| None` is applied to optional parameters when not appropriate

Appending `| None` if a doctype is marked as "optional" is only correct if the default is actually `None`. E.g. 
 
```python
def foo(a=None, b=1):
    """
    Parameters
    ----------
    a : str, optional
    b : int, optional
    """
```
should be transformed to
```python
def foo(a: str | None = None, b: int = ...) -> None: ...
```