Don't crash when receiving events from retail mobile app

My cron stopped running, which I traced to the process_paymentprocessor_webhooks job.

On closer examination, I was receiving a net.authorize.payment.authcapture.created event but $response->transaction->order didn't exist. I then saw the source was the iOS Retail App.

While supporting handling these events would be a bigger job, I made the essential change so as to not crash cron when these events are received.