`getClientListStacking` in WSL2 raises `TypeError: 'NoneType' object is not iterable`

```
  File "/home/avasam/.local/lib/python3.9/site-packages/ewmh/ewmh.py", line 257, in getClientListStacking
    return [self._createWindow(w)

TypeError: 'NoneType' object is not iterable
```

It seems `self._getProperty('_NET_CLIENT_LIST_STACKING')` returns `None` in a WSLg environment.