fix: use empty cancellation reason as default

[dankeboy36]

Let clients conveniently cancel the token without any reason.

---

The cancellation works very nicely. Thanks for the great work, Dennis!

This PR proposes an API change to let clients cancel the token without a reason.

Before:

useEffect(() => {
    const token = new CancellationTokenImpl();
    search(query, filter, token);
    return function () {
        token.cancel('');
    };
}, [search, filter, query]);

After:

useEffect(() => {
    const token = new CancellationTokenImpl();
    search(query, filter, token);
    return function () {
        token.cancel();
    };
}, [search, filter, query]);