primivitus: do not notify when an empty message is received

frontends/src/primitivus/chat.py

@@ -302,6 +302,8 @@ class Chat(urwid.WidgetWrap, QuickChat):
         @param from_jid: contact who wrote to the users
         @param msg: the message that has been received
         """
+        if msg == "":
+            return
         if self.text_list.get_focus()[1] == len(self.content) - 2:
             #we don't change focus if user is not at the bottom
             #as that mean that he is probably watching discussion history