Fix always-failing Invoice::upcoming(...)

[bryanburgers]

Invoice::upcoming(...) would always fail. The API call returns an
invoice that doesn't actually exist, and because it doesn't exist, it
doesn't have an "id" field in the response.

Note that when you are viewing an upcoming invoice, you are simply
viewing a preview – the invoice has not yet been created. As such, the
upcoming invoice will not show up in invoice listing calls, and you
cannot use the API to pay or edit the invoice. If you want to change the
amount that your customer will be billed, you can add, remove, or update
pending invoice items, or update the customer’s discount.

The lack of an "id" field causes the parsing of an Invoice to fail.

We can't do struct Invoice { id: Option<InvoiceId> } because the
Object trait expects the ID of the object to not be an option.

It seems like the least terrible option is to set #[serde(default)] on
the ID field and provide an empty string as an ID. While this isn't
great, it seems like the best of the bad options.

[bryanburgers]

Note: I also logged this over at stripe/openapi#56 (comment) and Stripe is looking into what to do about this from an openapi standpoint.

[kestred]

Thanks for logging an issue with stripe's openapi 😅 .

I'll merge this into both release/v0.12.x branch and the master (v0.13.0-dev) branch