[MERGE] forward port branch 11.0 up to bb6f6c57f9

addons/mail/models/mail_thread.py

@@ -191,6 +191,8 @@ class MailThread(models.AbstractModel):
     @api.multi
     def _get_message_needaction(self):
         res = dict((res_id, 0) for res_id in self.ids)
+        if not res:
+            return
 
         # search for unread messages, directly in SQL to improve performances
         self._cr.execute(""" SELECT msg.res_id FROM mail_message msg