remove_all_empty_atts.php

<?php

/*
 * Sometimes you might find a WooCommerce shop ends up with products that
 * have an attribute assigned but no terms selected for that attribute.
 * I used this script to bulk delete any empty attribute from 117 products.
 * In my case I was able to simply hook it to 'init', however when dealing
 * with 500 plus products this may cause a timeout error. In these cases
 * I would make this an AJAX function and create a JS script to handle multple
 * requests in chunks.
 */

//add_action( 'init', 'remove_all_empty_atts' );

function remove_all_empty_atts() {
    $products = get_posts(
        array(
            'post_type' => 'product',
            'posts_per_page' => -1,
        )
    );
	foreach ($products as $product) {
        $new_attributes = $old_attributes = get_post_meta( $product->ID, '_product_attributes', true );
		foreach ($old_attributes as $attribute => $values) {
			$post_terms = wp_get_post_terms( $product->ID, $attribute );
            if ( !is_wp_error( $post_terms ) ) {
                if ( empty( $post_terms ) ) {
                    unset($new_attributes[$attribute]);
                }
            }
        }
        update_post_meta( $product->ID, '_product_attributes', $new_attributes );
    }
}