By default, Yoast SEO does not add empty terms to the sitemap to prevent thin content issues. This code will allow empty terms to be added to the sitemap.

yoast_seo_sitemap_include_empty_terms.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/* Show all empty terms in Yoast SEO sitemaps
 * Credit: Yoast team
 * Last Tested: 03 Jan 2025 using Yoast SEO 24.1 on WordPress 6.7.1
 */
   
add_filter('wpseo_sitemap_exclude_empty_terms', '__return_false');