Make parameters optional

Because class properties in PHP can only be assigned a primitive type, the $parameters property on ReplicationTaskInterface and on ChangesInterface default to NULL.

When passing the parameters from ReplicationTaskInterface to ChangesInterface it's convenient to just do:

    $source_changes = $this->changesFactory->get($source_workspace)
        ->filter($task->getFilter())
        ->parameters($task->getParameters())
        ->getNormal();

But, in the above code, you cannot guarantee that $task->getParameters() is returning a ParameterBag.

So, I propose we make the $parameters parameter optional.