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

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

Add QUIQQER Tooling

See merge request !32

fix: shipping debug triggers Fatel Error

See merge request !40

fix: shipping debug triggers Fatel Error

See merge request !39

related: #54

fix: shipping rule does not open

See merge request !38

related: #52

fix(ShippingRules): remove buttons attribute from options

See merge request !36

Related: #52

fix: if linked entities, then do not set standard shipping

See merge request !35

If a standard shipping is set, it is not automatically set to erp entities if these have linked erp entities.
In this case, these erp entities have been created from the others. the shipping has then been changed, deleted, etc.

fix: call to undefined method addStatusMail

See merge request !34

fix: sendStatusChangeNotification allowes every erp entity

See merge request !33

Next

See merge request !31

fix: quiqqer/quiqqer locale and paths adapted to quiqqer/core

See merge request !30

feat!: quiqqer v2

Closes #51

See merge request !29

feat!: quiqqer v2

See merge request !28

feat!: quiqqer v2

See merge request !27

BREAKING CHANGE: quiqqer v2 has been renamed to quiqqer/core. a new major version will be created here so that we can still develop for both versions.