WooCommerce : Hide coupon code fields on checkout if no coupon publish.

tweak-woocommerce-hide-coupon-field.php

<?php 

add_filter( 'woocommerce_coupons_enabled', 'restofrais_coupons_enabled_checkout' );

if( ! function_exists('restofrais_coupons_enabled_checkout')){
	function restofrais_coupons_enabled_checkout($coupons_enabled){

		$args = array(
			'post_type' => 'shop_coupon',
			'post_status'      => 'publish',
		 );
		$postslist = get_posts( $args );

		if(empty($postslist)){
			return false;
		} else {
			return $coupons_enabled;
		}
	}
}