Enable strict nullability checks

[deffrian]

Right now we poorly handle nullable values because in some parts nullability checks are disabled.

[kamilchodola]

@rubo adjusted it recently for cli and env, right?

@deffrian Is it about configuration settings or sth else?

[rubo]

@kamilchodola No, this is about nullable annotations in C#, and the issue is still actual.

[LukaszRozmej]

I think only few test projects left?