Use NULL for generic marshallers in g_signal_new()

glib will use the correct marshaller automatically. And as a side
effect, we also get all glib optimizations, like a va marshaller.

gtk/gtktreesortable.c

@@ -86,7 +86,7 @@ gtk_tree_sortable_base_init (gpointer g_class)
                     G_SIGNAL_RUN_LAST,
                     G_STRUCT_OFFSET (GtkTreeSortableIface, sort_column_changed),
                     NULL, NULL,
-                    _gtk_marshal_VOID__VOID,
+                    NULL,
                     G_TYPE_NONE, 0);
       initialized = TRUE;
     }