[WebUI] Fix progress divide by 0 error with empty dir

If a dir exists with no contents then the following error occurred:

```
Traceback (most recent call last):
  ...
  File "/usr/lib/python3.10/site-packages/deluge/ui/web/json_api.py", line 608, in _on_got_files
    dirinfo['progress'] = sum(progresses) / dirinfo['size'] * 100
builtins.ZeroDivisionError: float division by zero
```

Closes: https://github.com/deluge-torrent/deluge/pull/439

deluge/ui/web/json_api.py

@@ -600,7 +600,10 @@ class WebApi(JSONComponent):
 
                 progresses = dirinfo.setdefault('progresses', [])
                 progresses.append(torrent_file['size'] * torrent_file['progress'] / 100)
+                if dirinfo['size'] > 0:
                     dirinfo['progress'] = sum(progresses) / dirinfo['size'] * 100
+                else:
+                    dirinfo['progress'] = 100
                 dirinfo['path'] = dirname
                 dirname = os.path.dirname(dirname)