Update coupon.php

Unset current coupon code if customer enters blank coupon code into field.  This allows the customer the ability to remove the coupon code from their cart.

upload/catalog/controller/checkout/coupon.php

@@ -49,6 +49,7 @@ public function coupon() {
 
 		if (empty($this->request->post['coupon'])) {
 			$json['error'] = $this->language->get('error_empty');
+			unset($this->session->data['coupon']);
 		} elseif ($coupon_info) {
 			$this->session->data['coupon'] = $this->request->post['coupon'];
 
@@ -62,4 +63,4 @@ public function coupon() {
 		$this->response->addHeader('Content-Type: application/json');
 		$this->response->setOutput(json_encode($json));
 	}
-}
+}