Regression: type inference can fall back to List[<nothing>]

Previously if mypy couldn't infer a type for a variable, it would fall back to a type such as `List[Any]` instead of `List[<nothing>]`. However, #8036 accidentally changed this, as can be seen from this example:

```py
x = []  # type: ignore
print(x)
x = []
reveal_type(x)    # List[<nothing>] (used to be List[Any])
```

We should revert to the old behavior.