plugin list interest: initialise extra with an empty dict if not set in registerPubsub

sat/plugins/plugin_exp_list_of_interest.py

@@ -98,6 +98,8 @@ class ListInterest(object):
         @param extra(dict, None): extra data, key can be:
             - thumb_url: http(s) URL of a thumbnail
         """
+        if extra is None:
+            extra = {}
         yield self.createNode(client)
         interest_elt = domish.Element((NS_LIST_INTEREST, u"interest"))
         interest_elt[u"namespace"] = namespace