Enable nullable for Fido2.Models

[dradovic]

I would be really helpful to annotate the nullability of the Fido2.Models types. In other words, to set <Nullable>enable</Nullable> in the Fido2.Models project.

This way, it would become clear for FIDO2 newbies to understand which information is optional and which one is required.

For example, in the demo, you have a line of code that stores a credential coming from MakeNewCredentialAsync:

DevicePublicKeys = new List<byte[]>() { success.Result.DevicePublicKey }

If both Fido2.Models and Demo were NRT enabled, this would pop up as a compilation error. Righteously so, because I don't think it make sense to store a list containing a null along the credential. So in this example, it would really help if RegisteredPublicKeyCredential.DevicePubliKey was declared as byte[]?. And I assume there's plenty more.

[iamcarbon]

We can't use required members yet (the net6.0 system.text.json source generator doesn't support them) -- so there's no clean way to annotate non-null members in the models library right now.

We should, however, continue to annotate nullable members inside of #nullable enable blocks. If we drop net6.0 when it goes out of support, it should be trivial to annotate the remaining members.

[dradovic]

Yes. I guess the first step should be to have <Nullable>enable</Nullable> in the Fido2.Models project (just as you already have it in the Fido2 project). This issue is about that.

And then in a later step, the require keyword can be used to make sure that props are correctly initialized.
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