gtkbuilder-menus: translation-domain can be NULL

GtkBuilder defaults its translation-domain to NULL, which is a perfectly
fine value for d_gettext() and friends.

https://bugzilla.gnome.org/show_bug.cgi?id=669638

gtk/gtkbuilder-menus.c

@@ -247,7 +247,7 @@ gtk_builder_menu_end_element (GMarkupParseContext  *context,
       state->string = NULL;
 
       /* do the translation if necessary */
-      if (state->translatable && state->parser_data->domain)
+      if (state->translatable)
         {
           const gchar *translated;