Would love to see this converted to use Nullable Reference Types

I am using nullable reference types on a Pulumi IaC project where I use the builder pattern. But find it difficult to get dotnet to figure out when items are not null.

The Builder Pattern is a delayed constructor.
The things it is supposed to build will be null until the `Build()` method has been executed.
But after `Build` they won't be null.

I try to help by using `MemberNotNull` attributes but that seems to just kick the can down the road only level.

Start by inspecting the builder implementation and its unit tests in BuilderTestSample; the issue provides no file paths. Trace the properties that remain null until Build() and the existing MemberNotNull annotations. Done means nullable reference types recognize those members as non-null after Build(), with the unit tests still passing.

Written by the indexing model from the issue text.