gtk_popover_menu_bar_set_menu_model(): Accept model == NULL

The description of gtk_popover_menu_bar_new_from_model() and gtk_popover_menu_bar_set_menu_model() says that model is nullable. It is not, because of g_return_if_fail (G_IS_MENU_MODEL (model)) in gtk_popover_menu_bar_set_menu_model().

(gtkmm-demo:12367): Gtk-CRITICAL **: 17:05:57.937: gtk_popover_menu_bar_set_menu_model: assertion 'G_IS_MENU_MODEL (model)' failed

This MR replaces the g_return_if_fail() with

g_return_if_fail (model == NULL || G_IS_MENU_MODEL (model));