When an order has zero price, mark it as paid immediately when created

There is a NullProvider that (only) supports orders with zero price. Direct zero price orders to that no matter what `event.provider` says.

Remember to update both implementations of `create_order`:

* [ ] `tickets_v2/optimized_server/models/order.py`
* [ ] `tickets_v2/utils/create_order.py`

While setting `cached_status` _might_ be enough, there is an inherent assumption that payment information is always conveyed by `PaymentStamp`s, so better make one.