Stripe sample addPaymentMethodDetails is missing some logic

See https://github.com/firebase/functions-samples/blob/7eb43734bb88fb9ba9e859ae558f72b185377f28/stripe/functions/index.js#L61

In that line, `paymentMethod.customer` returned from `stripe.paymentMethods.retrieve()` may be a string, null, or a `Customer` object. The code should treat each of those cases separately when creating the `setupIntent` from it. Noticed that while porting it over to Typescript.