`pathlib.PurePath.with_stem('')` promotes file extension to stem

# Bug report

```python
>>> pathlib.PurePath('foo.jpg').with_stem('').stem
'.jpg'
```

The `with_stem()` call should raise `ValueError`, as it's impossible for a filename to have an empty stem and a non-empty suffix.


### Linked PRs
* gh-114612
* gh-114613