Adds a "No Vendor" filter link to the WooCommerce Products admin screen for stores using WooCommerce Product Vendors.

filter-products-no-vendor.php

// Add custom view for products without a vendor
add_filter( 'views_edit-product', function( $views ) {
    $class = ( isset($_GET['no_vendor']) && $_GET['no_vendor'] == 1 ) ? 'current' : '';
    $url = add_query_arg( 'no_vendor', '1', admin_url( 'edit.php?post_type=product' ) );
    $views['no_vendor'] = "<a href='{$url}' class='{$class}'>No Vendor</a>";
    return $views;
});

// Modify the query to show products without vendor when selected
add_action( 'pre_get_posts', function( $query ) {
    if ( !is_admin() || !$query->is_main_query() ) {
        return;
    }

    if ( isset( $_GET['no_vendor'] ) && $_GET['no_vendor'] == 1 ) {
        $query->set( 'tax_query', array(
            array(
                'taxonomy' => 'wcpv_product_vendors',
                'operator' => 'NOT EXISTS'
            )
        ));
    }
});