Add [NotNull] attribute on the Should() method for object assertions

[0xced]

Compelling example:

[Theory]
[InlineData(true)]
[InlineData(false)]
public void Test1(bool currentUser)
{
    IPrincipal principal = currentUser ? new WindowsPrincipal(WindowsIdentity.GetCurrent()) : new ClaimsPrincipal();
    IIdentity? identity = principal.Identity;
    
    identity.Should().NotBeOfType<GenericIdentity>();
    identity.IsAuthenticated.Should().BeFalse();
}

Fixes #1115