Fix "TrySingle" signature with honest nullable annotations

[atifaziz]

There was a to-do comment pending since PR #582 that questioned whether we could be more honest with the signatures of TrySingle regarding T being null when cardinality isn't one. Starting with C# 9, it looks like we can finally address this with unconstrained type parameter annotations:

In C#8, ? annotations could only be applied to type parameters that were explicitly constrained to value types or reference types. In C#9, ? annotations can be applied to any type parameter, regardless of constraints.

This PR fixes the signatures to be more honest and removes the need for the use of the dammit (!) operator within the implementation body.

We still can't express that T isn't null when cardinality is one (e.g. through some of the helper attributes for the compiler) but the new annotations are far better than what was there before.

[atifaziz]

@viceroypenguin Thanks for the review!