Update StripeGateway.php

If the customer has no card on file, don't require it.

Usecase: Customer is already registered on Stripe without a credit card but has a coupon on their account for 100% off or the cost of the plan your assigning them to, you want to subscriotn(plan)->swap() but currently it throws an error as it tried to get the default card on the account (which there is none).