message dialog: Prevent empty title from being shown

We don't want to grow the titlebar by showing an empty label.

gtk/gtkmessagedialog.c

@@ -442,6 +442,7 @@ gtk_message_dialog_constructed (GObject *object)
       gtk_widget_show (box);
       gtk_widget_set_size_request (box, -1, 16);
       label = gtk_label_new ("");
+      gtk_widget_set_no_show_all (label, TRUE);
       gtk_widget_set_margin_top (label, 6);
       gtk_widget_set_margin_bottom (label, 6);
       gtk_style_context_add_class (gtk_widget_get_style_context (label), "title");