Fix error in disco add_item.

None values were not being treated properly.

sleekxmpp/plugins/xep_0030/disco.py

@@ -345,7 +345,7 @@ class xep_0030(base_plugin):
         """
         self._run_node_handler('del_items', jid, node, kwargs)
 
-    def add_item(self, jid=None, name='', node=None, subnode='', ijid=None):
+    def add_item(self, jid='', name='', node=None, subnode='', ijid=None):
         """
         Add a new item element to the given JID/node combination.
 
@@ -359,10 +359,12 @@ class xep_0030(base_plugin):
             subnode -- Optional node for the item.
             ijid   -- The JID to modify.
         """
+        if jid is None:
+            jid = ''
         kwargs = {'ijid': jid,
                   'name': name,
                   'inode': subnode}
-        self._run_node_handler('add_item', jid, node, kwargs)
+        self._run_node_handler('add_item', ijid, node, kwargs)
 
     def del_item(self, jid=None, node=None, **kwargs):
         """
@@ -604,3 +606,4 @@ class xep_0030(base_plugin):
                           "Using default disco#info feature.")
                 info.add_feature(info.namespace)
         return info
+