This commit fixes the errors arising from non-existing classes in the AbstractOrder.php

Changes include:
- Added a comment on the shipping validation
- Corrected type declaration in the reversal function signature to include the possibility of the
parameter being null.
- Checked for the existence of ShippingStatus and ShippingEntry classes before attempting to get
Shipping status and Shipping Ids, respectively. This prevents errors if these classes do not exist.

Related: #172