quick_frontend (contact list): fixed `nick` use for groupchat: during the profile connection workflow, joined rooms are retrieved, then presence are filled, all asynchronously. In some cases, all rooms may not be joined when presence are filled, resulting in cache being already filled with a `nick` when cache is set for the room itself. Because of that, a `nick` may be displayed in contact list instead of MUC local part. This patch fixes it by removing `nick` if it exists when cache a MUC room. fix 305