Fix PHPDoc on Discount for nullable properties (stripe#1274)

lib/Discount.php

@@ -5,17 +5,17 @@
 /**
  * Class Discount.
  *
- * @property string $checkout_session The Checkout session that this coupon is applied to, if it is applied to a particular session in payment mode. Will not be present for subscription mode.
+ * @property null|string $checkout_session The Checkout session that this coupon is applied to, if it is applied to a particular session in payment mode. Will not be present for subscription mode.
  * @property \Stripe\Coupon $coupon Hash describing the coupon applied to create this discount.
  * @property string|\Stripe\Customer $customer The ID of the customer associated with this discount.
- * @property int $end If the coupon has a duration of repeating, the date that this discount will end. If the coupon has a duration of once or forever, this attribute will be null.
+ * @property null|int $end If the coupon has a duration of repeating, the date that this discount will end. If the coupon has a duration of once or forever, this attribute will be null.
  * @property string $id The ID of the discount object.
- * @property string $invoice The invoice that the discount’s coupon was applied to, if it was applied directly to a particular invoice.
- * @property string $invoice_item The invoice item id (or invoice line item id for invoice line items of type=‘subscription’) that the discount’s coupon was applied to, if it was applied directly to a particular invoice item or invoice line item.
+ * @property null|string $invoice The invoice that the discount’s coupon was applied to, if it was applied directly to a particular invoice.
+ * @property null|string $invoice_item The invoice item id (or invoice line item id for invoice line items of type=‘subscription’) that the discount’s coupon was applied to, if it was applied directly to a particular invoice item or invoice line item.
  * @property string $object String representing the object’s type. Objects of the same type share the same value.
- * @property string $promotion_code The promotion code applied to create this discount.
+ * @property null|string $promotion_code The promotion code applied to create this discount.
  * @property int $start Date that the coupon was applied.
- * @property string $subscription The subscription that this coupon is applied to, if it is applied to a particular subscription.
+ * @property null|string $subscription The subscription that this coupon is applied to, if it is applied to a particular subscription.
  */
 class Discount extends StripeObject
 {