Documentation shows unexpected ForwardRef(None) inside Union.

Input:
```
from typing import Union

def test1_func(arg: Union[None, str, int]) -> None:
  return
```

Resulting documentation:
`def test1_func(arg: Union[ForwardRef(None), str, int]) ‑> None`

Expected documentation:
`def test1_func(arg: Union[None, str, int]) ‑> None`

!pdoc --version
pdoc 0.10.0
