fix(phpstan): allow null as parameter in various functions

This commit does the following changes:

1. Updated the PHPStan version from 1.11.8 to 1.12.13 in phars.xml
2. In various classes, added the possibility to pass null as a parameter to a number of functions. Affected classes include but are not limited to: ShippingInterface, Handler, Status, StatusUnknown, and ShippingEntry.
3. Updated return types in the ShippingEntry class to include null option.
4. Simplified return types in various places from datatype|datatype|null to possibly nullable datatype.

PHP9 + PHP8.4 null deprecated