fix: PolyType [TypeShape] attribute and disable NRTs for the marshaler

[aradalvand]

Please see AArnott/Nerdbank.MessagePack#651 — goes back to #835

This PR fixes two things:

1. Currently, the PolyType integration doesn't even work with Nerdbank.MessagePack — and the culprit is Kind = TypeShapeKind.None within the [TypeShape] attribute. I removed that.
2. The generated PolyType marshaler causes a whole bunch of nullability warnings wherever a [ValueObject]'s underlying type is a reference type (e.g. string) — I wrapped the marshaler class in a #nullable disable/#nullable restore pair of preprocessors.

I also made the marshaler class sealed to improve perf a little bit as well.

[aradalvand]

@SteveDunn Could you please take a look here Steve?

[aradalvand]

Just updated the snapshot. The pipeline should pass now, hopefully.

[SteveDunn]

Hi - thank you for the PR. Would you mind resetting all of the snapshot files by running .\RunSnapshots.ps1 -v "minimal" -reset (and then go on a short holiday while it does its stuff! ;) )

[aradalvand]

Thank you @SteveDunn — that helped. There was a missing parenthesis I needed to sort out.