disable coupon button if the field is empty (https://github.com/woocommerce/woocommerce-blocks/pull/1977)

plugins/woocommerce-blocks/assets/js/base/components/totals/totals-coupon-code-input/index.js

@@ -76,7 +76,7 @@ const TotalsCouponCodeInput = ( {
 						/>
 						<Button
 							className="wc-block-coupon-code__button"
-							disabled={ isLoading }
+							disabled={ isLoading || ! couponValue }
 							onClick={ ( e ) => {
 								e.preventDefault();
 								onSubmit( couponValue );