Handle qButtorrent empty url-list/webseeds

url-list is supposed to be omitted in case of no webseeds,
but qB will instead use an empty bytestring '' as the value.
This commit makes the validation more lenient regarding url-list.

nyaa/forms.py

@@ -346,7 +346,8 @@ def _validate_trackers(torrent_dict, tracker_to_check_for=None):
 # http://www.bittorrent.org/beps/bep_0019.html
 def _validate_webseeds(torrent_dict):
     webseed_list = torrent_dict.get('url-list')
-    if webseed_list is not None:
+    # qBittorrent has an empty field instead of omitting the key, so just check for truthiness
+    if webseed_list:
         _validate_list(webseed_list, 'url-list')
 
         for webseed_url in webseed_list: