fix(phpstan): adjust method argument typings for better code readability

Updated method argument typings from `Type $var = null` to `null | Type $var = null` across
multiple files to improve readability and understanding of the code. This change aligns with modern
coding standards and the principle of explicit over implicit.
Affected files:
- `AbstractShippingEntry.php`
- `AbstractShippingType.php`
- `ShippingInterface.php`
- `ShippingTypeInterface.php`
- `Factory.php`
- `ShippingRule.php`
- `Shipping.php`

PHP9 + PHP8.3 null deprecated