Fix serialization of enums with Null values

[andreas-hilti]

Closes: #445

The [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)] prevents writing the Null value in the Json case, while the ShouldSerialize... member prevents it in the Xml case.

[andreas-hilti]

There are a couple of enums that I haven't touched:

• if they are used in lists: List<CertificationLevel>, List<CryptoFunction>, List<Response>
• if they are used as nullable: AlgorithmMode?, ExecutionEnvironment?, ImplementationPlatform?, etc. (only partially addressed)

There are also a couple of instances where the enums are required, e.g. DataClassification.Flow; there I prevent writing the "Null" value, but this doesn't guarantee a valid JSON, given that a proper enum value is required.