Fix some nullable reference type errors in Visual Studio 16.8

[bamarsha]

It looks like there were some changes to nullable reference type analysis in Visual Studio 16.8, causing some errors when building the Q# compiler. (This is despite having a global.json that sets the SDK version to 3.1 - dotnet build would succeed, but building in VS itself would fail.) The changes needed to fix the errors continue to work in 3.1, and mostly seem reasonable.

There's one (small) breaking change. I removed the default parameter from the TypedExpressionWalker constructor:

public TypedExpressionWalker(Action<TypedExpression> onExpression, T internalState = default)

This could be worked around with nullable attributes or the null-forgiving operator, but having a null transformation state does not seem like a great idea, anyway.

[filipw]

possibly because it was not set to 3.1.403?

[bamarsha]

No, changing the version in global.json to 3.1.403 or 3.1.404 doesn't seem to work. I think the global.json is working, because dotnet build uses 3.1, but the analyzer/compiler built into Visual Studio seems to have changed behavior slightly.