message_list: Update trailing bookend on empty channel or topic.

Show `You are not subscribed to  #xyz. Subscribe` bookend
on channel or topic which are not subscribed and have no messages.

Fixes: #33209

web/src/message_fetch.ts

@@ -127,6 +127,7 @@ export function fetch_more_if_required_for_current_msg_list(
         // Even after loading more messages, we have
         // no messages to display in this narrow.
         narrow_banner.show_empty_narrow_message(message_lists.current.data.filter);
+        message_lists.current.update_trailing_bookend();
         compose_closed_ui.update_buttons_for_private();
         compose_recipient.check_posting_policy_for_compose_box();
     }