[13.x] Use isEmpty() method instead of redundant count() checks in FakeProcessSequence

[sumaiazaman]

Currently, there are places where we check count($this->processes) === 0 directly. However, the class already has a dedicated method isEmpty() that performs the same check:

public function isEmpty()
{
    return count($this->processes) === 0;
}

This PR updates the code to use isEmpty() instead of direct count() checks. This improves readability and ensures consistency, as the isEmpty() method is already used in multiple places.