show a warning when `Unknown` is inferred

```py
class A:
a = None # expect a warning or `None`, not `None | Unknown`
```

Start with the type-inference path exercised by the `class A` example and trace how `None` becomes `None | Unknown`. Confirm the intended behavior in the existing checker tests, then ensure this case produces a warning or infers `None` without `Unknown` and add coverage for the example.

由索引模型根据 Issue 内容生成。