Add nullable context

[ThierryRosseau]

Add nullable context to generated interface to fix warning

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source. 

[ThierryRosseau]

@daver32 @adamhathcock Can this be validated and merged if possible with a new build? Thx very much

[adamhathcock]

I don't have access to this repo but I'd add it if I could.

Lack of nullability is holding this back