How to Emulate Default Navigation Menu Behaviour in a Custom Theme?

The default wp_nav_menu arguments set the fallback_cb argument to wp_page_menu, which outputs a list of pages if your menu doesn’t exist. If your menu doesn’t behave this way, that argument has been overridden in some way, either from being explicitly set in the wp_nav_menu call, or perhaps via the wp_nav_menu_args filter.