WooCommerce - Show empty product categories.

functions.php

<?php
/**
 * WooCommerce - Show empty product categories.
 *
 * @param  array $args Default widget argument.
 *
 * @return array       New arguments with hide_empty.
 */
function wc_show_empty_categories( $args ) {
    $args['hide_empty'] = 0;

    return $args;
}

add_filter( 'woocommerce_product_categories_widget_args', 'wc_show_empty_categories' );