Introduce has_nav_menu filter

[westonruter]

For Menu Customizer (#32576) to preview changes with partial refresh, we need to know which menu locations are used on a given template. In Twenty Fifteen, the wp_nav_menu calls are wrapped in a has_hav_menu() checks, meaning that the hooks that fire in wp_nav_menu will not be triggered if there was no menu assigned to the location up-front. So in the same way that we have a is_sidebar_active() function and a corresponding is_active_sidebar filter, we also should have a has_nav_menu filter accompany the has_hav_menu() function.

[westonruter]

Source: ​https://github.com/xwp/wordpress-develop/pull/91/files#diff-8ffabe2d2470a815389fd77c045d2c7fL148

[ocean90]

In 32746:

Nav menu: Introduce a has_nav_menu filter in has_nav_menu() to filter whether a nav menu is assigned to a specified location.

props westonruter.
fixes #32630.

[akibjorklund]

#31525 was marked as a duplicate.

[swissspidy]

#31525 was marked as a duplicate.