Document nullish timer argument for cancel()

Current documentation doesn't mention that the timer argument can be undefined/null:
https://api.emberjs.com/ember/4.1/functions/@ember%2Frunloop/cancel

However, the implementation allows that:
https://github.com/BackburnerJS/backburner.js/blob/af77b18f37ec24e89f65bef0ce16bc3c8d1f4bfb/lib/index.ts#L551

In fact, in our code, we were leveraging the fact that you can pass undefined as the timer argument. It may be useful if you have a local var where you save the timer, so, the type f this var is `EmberRunTimer | undefined` because the timer is not initialized in the constructor. In `willDestroy` hook we call `cancel(this.timer)`.

In fact, we should understand if we can keep doing that or we cave to wrap such calls with `if (this.timer)` check.

@ember/runloop types do not allow undefined, this is where this issue came from. https://github.com/DefinitelyTyped/DefinitelyTyped/pull/58893