Fixed problem with finding nullable attribute in nested types because of roslyn optimizations

[Eneuman]

This change only affects the logic in finding compiler annotated null attributes when using the "SupportNonNullableReferenceTypes" option.

• Adjusted code to handle roslyn compiler optimizations for nested types.
• Fixed bug that cause a crash if annotation nullable attribute flag returned was byte[].
• Added tests for compiler optimization senarios.
• Added default values to test class to remove warnings.

More information about roslyn nullable metadata can be found here https://github.com/dotnet/roslyn/blob/main/docs/features/nullable-metadata.md

This PR should fix #2017 and probobly #1487