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