Example should be updated with correct nullable notation

The example will produce several warning regarding the use of nullable notation if copied into a new .net 6 project with default settings. The example should be updated to eliminate all the warnings.
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#### Document Details

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* ID: d118f070-8743-e905-2d08-dc725a4e8b5f
* Version Independent ID: fae35cbd-75ae-4448-f066-7c91ccc35779
* Content: [Using Enumeration classes instead of enum types](https://learn.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/enumeration-classes-over-enum-types)
* Content Source: [docs/architecture/microservices/microservice-ddd-cqrs-patterns/enumeration-classes-over-enum-types.md](https://github.com/dotnet/docs/blob/main/docs/architecture/microservices/microservice-ddd-cqrs-patterns/enumeration-classes-over-enum-types.md)
* Product: **dotnet-architecture**
* Technology: **microservices**
* GitHub Login: @jamesmontemagno
* Microsoft Alias: **jamont**