I actually think I have managed to find a solution to this after a lot of Googling 😂, looks like I didn’t need to know how the theme was querying the categories.
Used this code in case anyone else comes across this thread:
add_filter( 'wp_get_nav_menu_items', 'nav_remove_empty_category_menu_item', 10, 3 );
function nav_remove_empty_category_menu_item ( $items, $menu, $args ) {
global $wpdb;
$nopost = $wpdb->get_col( "SELECT term_taxonomy_id FROM $wpdb->term_taxonomy WHERE count = 0" );
foreach ( $items as $key => $item ) {
if ( ( 'taxonomy' == $item->type ) && ( in_array( $item->object_id, $nopost ) ) ) {
unset( $items[$key] );
}
}
return $items;
}