Fixed errors for non-subscription invoices

refs https://github.com/TryGhost/Team/issues/887

Our invoice webhook handling code assumed that every invoice would be
for a subscription, but that is not the case. There are valid use-cases
of using the same Stripe account in order to sell items with a one-off
purchase. Here we update the handling to ignore all invoices which are
not for subscriptions.

ghost/members-api/lib/services/stripe-webhook.js

@@ -155,6 +155,9 @@ module.exports = class StripeWebhookService {
      * @returns {Promise<void>}
      */
     async invoiceEvent(invoice) {
+        if (!invoice.subscription) {
+            return;
+        }
         const subscription = await this._stripeAPIService.getSubscription(invoice.subscription, {
             expand: ['default_payment_method']
         });