add_filter( 'woocommerce_product_query_meta_query', 'shop_only_instock_products', 10, 2 );
function shop_only_instock_products( $meta_query, $query ) {


    if( is_admin() ) return $meta_query;

    $meta_query['relation'] = 'OR';

    $meta_query[] = array(
        'key'     => '_price',
        'value'   => '',
        'type'    => 'numeric',
        'compare' => '!='
    );
    $meta_query[] = array(
        'key'     => '_price',
        'value'   => 0,
        'type'    => 'numeric',
        'compare' => '!='
    );
    return $meta_query;
}