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

Added a conditional check in src/QUI/ERP/Order/Basket/Basket.php to convert basketId to actual id
from user's session if it is a boolean value. This fixes potential issues that may arise with
non-integer basketIds.

Also, introduced a safeguard check before saving to ensure there is a valid User. If not, the save
function is exited early to prevent potential errors or undesired actions.

related: quiqqer/order-guestorder#8

feat: merge shopping cart and order if products are different. if products were placed in the shopping cart in the status not logged in

feat: user currency -> the user can now have his own currency, so he can use the exchange rate to display the price in his currency.