Add support for ReturnsNull calls for nullable value types

[Stedoss]

Currently, you are not able to ReturnsNull() on nullable value types (eg. int? and Guid?), and when wrapped in tasks they will throw a nullref, such as:

var foo = Substitute.For<IFoo>();
foo.Bar().ReturnsNull();
Console.WriteLine(await foo.Bar());

public interface IFoo
{
    public Task<int?> Bar();
}

This PR should help fix this by extending the extension methods to handle nullable value types, along with wrapping them in Task and ValueTask types when necessary.

I have written unit tests such that it tests for nullability on the newly added functionality, please let me know if more thorough tests are needed.

Note: ReturnsNull() will still throw a nullref if the value type inside of a Task is NOT nullable.

Thank you!

[dtchepak]

Thanks @Stedoss !

[dtchepak]

Released in 4.4.0.