Feature: Nullable reference types annotations

[hez2010]

We are migrating to WASDK and adopt .NET 6, so we get build-in nullable reference types support.

Requirements

• We should annotate our code to eliminate thousands of warnings about NRT.

This is a large one, but we can do it progressively.

[yaira2]

Good idea, I believe we can do this one by one (e.g. update things as you notice them).