Improve readability and interpretability of code by explicitly stating null in union types. This
change has been applied across multiple files and functions.

PHP8.4 compatibility

feat: new payment processing api finished; js and php has now the possibility to react at payment processing errors

feat: payment changeable -> payment methods can be changed after the order process, if there was an error during the payment process

- complete ordering process and the individual steps made functional
- processing step implemented
- processing step over considered everywhere