fix: update `getCustomerFiles` method signature in `AbstractOrder`

Changes include:
- Updated `AbstractOrderingStep.php` to handle null case in `getOrder` method and modified return
type.
- Refactored `Order.php` to remove unnecessary checks and improve readability.
- Ensured safety against calling non-existent `save()` method in `Checkout.php`.
- Simplified address getting logic in `CustomerData.php`.
- Removed unnecessary check in isUserB2B function.

Related: #172