list overloads don't handle nullability correctly

```python
    class Post:
        user = fields.ListField(fields.StringField())

    post = Post()
    # should be 'list[str | None] | None' since user is not required and StringField is not required.
    reveal_type(post.user) # Revealed type is 'list[str]'
```

related: https://github.com/python/mypy/issues/10312