Sort posts by optional meta key then by post date

// filter the posts clauses for some shenanigans
// order by an option meta field and then by post date
add_filter( 'posts_clauses', 'kia_edit_posts_clauses', 10, 2 );

function kia_edit_posts_clauses( $pieces, $query ) {

    if ( $query->get( 'orderby' ) == 'dealer_date' && isset( $query->query_vars['query_id'] ) && $query->query_vars['query_id'] == "woocommerce_my_account_my_orders_query" ) {

            global $wpdb;

            // join the post_meta table
            $pieces[ 'join' ] .= $wpdb->prepare( ' LEFT JOIN ' . $wpdb->postmeta . ' dealer_pm ON dealer_pm.post_id = ' . $wpdb->posts . '.ID AND dealer_pm.meta_key = %s AND LENGTH(TRIM(dealer_pm.meta_value) )', '_shipping_dealer_number' );

            // Negate the meta_value if it exists to sort before zero.
            $pieces[ 'fields' ] .= ', CASE WHEN dealer_pm.meta_value IS NOT NULL THEN -dealer_pm.meta_value ELSE 0 END AS dealer_number';

            $pieces[ 'orderby' ] = $pieces[ 'orderby' ] = 'ISNULL(MAX(dealer_pm.meta_value)) OR LENGTH(TRIM(MAX(dealer_pm.meta_value))) = 0 ASC, MAX(dealer_pm.meta_value) ASC,' . $wpdb->posts . '.post_date DESC';

        }

        return $pieces;
    }


// then my query
$customer_orders = get_posts( apply_filters( 'woocommerce_my_account_my_orders_query', array(
    'query_id' => 'woocommerce_my_account_my_orders_query',
    'posts_per_page' => $order_count,
    'no_found_rows' => true,
    'date_query' => array(
        array(
            'after' => '2 months ago',
            ),
    ),
    'meta_query' => array(
        array(
            'key'     => '_customer_user',
            'value'   => get_current_user_id(),
            'compare' => '=',
        ),
    ),
    'post_type'   => wc_get_order_types( 'view-orders' ),
    'post_status' => array_keys( wc_get_order_statuses() ),
    'suppress_filters' => false,
    'orderby' => 'dealer_date'
) ) );