This update adjusts how Odoo handles tax exemption reasons on UBL documents, aligning with updated regulations. Now, if a tax exemption reason isn't provided, a default reason is automatically applied for relevant tax categories, ensuring compliance and accurate reporting. This simplifies the process for users and improves data consistency.

According to the ubl documentation the tax exemption reason code is not
always required on the document. But when no exemption reason code is 
given, we have a default exemption reason for the appropriate tax 
categories.


task: 5223145

Forward-Port-Of: odoo/odoo#242055
Forward-Port-Of: odoo/odoo#233770