None is not a possible value for type variable bound to optional value?!

```python
from typing import TypeVar

class X: pass
U = TypeVar('U', bound=None | X)

def project(projectable: U) -> U:
    if projectable is None:
        return None  # MyPy claims this is unreachable!
    return projectable
```
If warn-unreachable is enabled, then MyPy claims this is unreachable.