Fixing notice when trying to set empty string

[IgorNadj]

This is to fix an error, found with this stacktrace:

ERROR [Notice]: Undefined index: 
IN POST /admin/discounts/OrderCoupon/GenerateCouponsForm
Line 324 in /var/www/public/shop_discount/code/model/Discount.php

Source
======
  315:   * @param string $val
  316:   */
  317:  public function setFor($val){
  318:      $map = array(
  319:          "Items" => array(1,0,0),
  320:          "Cart" => array(0,1,0),
  321:          "Shipping" => array(0,0,1),
  322:          "Order" => array(0,1,1)
  323:      );
* 324:      $mapping = $map[$val];
  325:      $this->ForItems = $mapping[0];
  326:      $this->ForCart = $mapping[1];
  327:      $this->ForShipping = $mapping[2];
  328:  }
  329: 
  330:  public function getFor(){

Trace
=====
Discount->setFor()
ViewableData.php:125

ViewableData->__set(For,)
DataObject.php:2375

DataObject->setCastedField(For,)
Discount.php:263

Discount->setCastedField(For,)
FormField.php:217

FormField->saveInto(OrderCoupon)
Form.php:1271

Form->saveInto(OrderCoupon)
DiscountModelAdmin.php:130

DiscountModelAdmin->generate(Array,Form,SS_HTTPRequest)
Form.php:370

Form->httpSubmission(SS_HTTPRequest)
RequestHandler.php:288

RequestHandler->handleAction(SS_HTTPRequest,httpSubmission)
RequestHandler.php:200

RequestHandler->handleRequest(SS_HTTPRequest,DataModel)
RequestHandler.php:222

RequestHandler->handleRequest(SS_HTTPRequest,DataModel)
Controller.php:153

Controller->handleRequest(SS_HTTPRequest,DataModel)
LeftAndMain.php:441

LeftAndMain->handleRequest(SS_HTTPRequest,DataModel)
AdminRootController.php:93

AdminRootController->handleRequest(SS_HTTPRequest,DataModel)
Director.php:366

Director::handleRequest(SS_HTTPRequest,SkinnySession,DataModel)
Director.php:152

Director::direct(/admin/discounts/OrderCoupon/GenerateCouponsForm,DataModel)
main.php:190

[markguinn]

Thanks!