`-x` and `-r` flags don't work with GTK 4.0

When running `nwg-look -x` or `nwg-look -r`, the gtk-4.0 files aren't symlinked and the following warning is emitted:

```
WARN[0000] Unknown theme path: ''
```

I made a small fix for this and will open a PR linked to this issue.

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Thanks for your time
