[Bug]: Per OpenAPI Non-nullable enum can be indeed null

[Baumbart13]

What happened?

Get checkouts
GET https://api.sumup.com//v0.1/merchants/{merchant_code}/readers/{reader_id}/checkout/{checkout_id}

The return value of GetReaderCheckoutResponseDataCardType can happen to be null, but the property CardType in GetReaderCheckoutResponseData gets generated as a non-nullable enum.

Content of the JSON:
{"data":{"card_type":null,"checkout_id":"8cd0fc7e-e1f1-462a-b6f5-14b924521aba","client_transaction_id":"8390f4b5-2a65-499b-bd14-27d7b4002bfd","created_at":"2026-08-18T15:08:20.242357Z","installments":null,"payment_failure_reason":null,"payment_status":null,"payment_type":"card","reader_firmware_version":"v0.1.0","reader_serial_number":"virtual_solo_e8a00ef0-6d38-480d-bf3c-e233b3223693","status":"pending","total_amount":{"currency":"EUR","minor_unit":2,"value":1000},"updated_at":"2026-08-18T15:08:20.256784Z","valid_until":"2026-08-18T15:09:20.242176Z"}}

Relevant code or error output

var client = new SumUpClient();
var readerCheckoutCreated = await client.Readers.CreateCheckoutAsync(merchantCode, readerId, request, requestOptions);
var readerCheckoutFetched = await client.Readers.GetCheckoutAsync(merchantCode, readerId, readerCheckoutCreated.Data.Data.CheckoutId, requestOptions);

[ERR] System.Text.Json.JsonException: Unable to convert value to enum type 'SumUp.GetReaderCheckoutResponseDataCardType'.
   at SumUp.EnumMemberJsonConverterFactory.EnumMemberJsonConverter`1.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options) in C:\prog\repos\sumup-dotnet-baumbart13\src\SumUp\EnumMemberJsonConverterFactory.cs:line 90

SDK version

0.0.18

.NET SDK or runtime version

.NET 8

Target framework

net8.0

Operating system

Windows 11

Additional context

No response