Don't crash if there is no address to associate

Rather, just to nothing. This fixes
https://www.honeybadger.io/inspector/p/33720/5871315
Default credit card per user

When an order is completed, we save the credit card used as the user's
default credit card. When creating a new order, we use an existing
credit card (if it exists) and attach it to the order before reaching
the payment state.