Prevent wp_get_nav_menu_items filter from not applying if there are no menu items

[westonruter]

For Menu Customizer (#32576) we need to filter a menu's items. The menu may not have any items to begin with. The filter that is most useful for the filtering is wp_get_nav_menu_items but this is currently not getting called if there were no items found up front. We need to change this so that even if no items were found up front, that the filter can still apply so plugins can add their own items to be returned.

[westonruter]

Source: ​https://github.com/xwp/wordpress-develop/pull/91/files#diff-8ffabe2d2470a815389fd77c045d2c7fR562

[ocean90]

In 32748:

Nav menus: Apply the wp_get_nav_menu_items filter also on an empty list of menu items.

props westonruter.
fixes #32631.