Copy the WooCommerce order billing address to user profile when profile email is blank

functions.php

<?php

add_action( 'woocommerce_checkout_order_processed', 'my_woocommerce_copy_billing_email_to_user_profile', 10, 3 );

/**
 * Copy the order billing address to user profile when profile email is blank.
 *
 * Requires WooCommerce 3.0
 *
 * @param $order_id
 * @param $posted_data
 * @param \WC_Order $order
 */
function my_woocommerce_copy_billing_email_to_user_profile( $order_id, $posted_data, $order ){
	$user = $order->get_user();

	if ( ! $user || $user->user_email ) {
		return;
	}

	$user->user_email = $order->get_billing_email();

	wp_update_user( $user );
}