PLC1901 false positive in `__bool__` methods

```py
class X:
    val: str

    def __bool__(self) -> bool:
        return self.val != ""
```
```sh
$ ruff check test.py --select=PLC1901
test.py:5:28: PLC1901 `self.val != ""` can be simplified to `self.val` as an empty string is falsey
Found 1 error.
```
Trying to correct this to `self.val` will lead to an error:
```
TypeError: __bool__ should return bool, returned str
```
