subscription_proration_date not always presented in Invoice (#1236)

lib/Invoice.php

@@ -108,7 +108,7 @@
  * @property null|string $status The status of the invoice, one of <code>draft</code>, <code>open</code>, <code>paid</code>, <code>uncollectible</code>, or <code>void</code>. <a href="https://stripe.com/docs/billing/invoices/workflow#workflow-overview">Learn more</a>
  * @property \Stripe\StripeObject $status_transitions
  * @property null|string|\Stripe\Subscription $subscription The subscription that this invoice was prepared for, if any.
- * @property int $subscription_proration_date Only set for upcoming invoices that preview prorations. The time used to calculate prorations.
+ * @property null|int $subscription_proration_date Only set for upcoming invoices that preview prorations. The time used to calculate prorations.
  * @property int $subtotal Total of all subscriptions, invoice items, and prorations on the invoice before any invoice level discount or exclusive tax is applied. Item discounts are already incorporated
  * @property null|int $subtotal_excluding_tax The integer amount in %s representing the subtotal of the invoice before any invoice level discount or tax is applied. Item discounts are already incorporated
  * @property null|int $tax The amount of tax on this invoice. This is the sum of all the tax amounts on this invoice.