Changed usage of CancellationToken

[arkada38]

We can use
CancellationToken cancellationToken = default
instead of
CancellationToken? cancellationToken = null and cancellationToken ?? CancellationToken.None

using System.Diagnostics;
using System.Threading;

CancellationToken cancellationToken = default;
Debug.Assert(cancellationToken == CancellationToken.None);

CancellationToken? cancellationToken = null;
cancellationToken = CancellationToken.None;
Debug.Assert(cancellationToken == CancellationToken.None);