don't send thread ids in message receipts unless there's a thread

src/common/connection_handlers.py

@@ -1676,7 +1676,9 @@ class ConnectionHandlers(ConnectionVcard, ConnectionBytestream, ConnectionDisco,
 			receipt.setID(msg.getID())
 			receipt.setTag('received',
 				namespace='urn:xmpp:receipts')
-			receipt.setThread(thread_id)
+
+			if thread_id:
+				receipt.setThread(thread_id)
 			con.send(receipt)
 
 		if mtype != 'groupchat':