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// removes Menu-Items if they got no posts - tested with portfolio and posts
// put it into child-theme functions.php
 
function remove_empty_category_menu ( $items, $menu, $args ) {
    if ( ! is_admin() ) { 
        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;
}
add_filter( 'wp_get_nav_menu_items', 'remove_empty_category_menu',10, 3 );