Private file system path should not be reported unless configured

Prior to this patch, drush status would report that the private file system path == the public file system path if the private path is not set. With this patch, the private file system path is not reported in status unless it is set.

Not reported is better than empty, because this way the drupal-directory command will similarly return 'no such path' rather than returning the Drupal root (which it would do if we made this default to empty rather than unset).