尝试将查询参数修改为：

    $options[\'ind_start\'] = array(
        \'label\' => \'Start Date (Earliest)\',
        \'query_args\' => array(
            \'meta_query\' => array(
                \'start_date_query\' => array(
                    \'key\' => \'start_date\',
                    \'compare\' => \'EXISTS\', // Optional
                ),
            )
        )
    );

meta_query - 将只包括带有start_date 价值

然后添加WP查询顺序过滤器：

function my_order_filter($orderby, $obj){
    global $wpdb;

    // Put here some conditional logic if you want to apply this only for a specific query.

    $new_order_query_part = "{$wpdb->postmeta}.meta_value = \'-\' ASC, {$wpdb->postmeta}.meta_value ASC ";
    if( trim($orderby) ){
        $orderby = $new_order_query_part . \', \' . $orderby;
    } else {
        $orderby = $new_order_query_part;
    }

    return $orderby;
}
add_filter( \'posts_orderby_request\', \'my_order_filter\', 10, 2 );

此筛选器将应用于所有查询。如果只需要将订单功能添加到特定查询，那么在将新订单应用于请求之前，请使用一些条件筛选。