This fix resolves an issue where the currency information from purchase orders was not being imported correctly when orders were received through EDI or OCR systems. The problem occurred because a section header was being added to the invoice before the purchase order data was imported, which prevented the currency from being recognized. This update ensures that currency information is properly captured during the automated import process.

When a purchase order is imported through an EDI or the OCR, a line section "From PXXXXX document" is inserted in the lines before the actual import of the PO content.

This had the side effect to disable the import of the currency from the PO because it's only imported if the invoice doesn't already contain invoice lines.

Related ticket: 3756004

Forward-Port-Of: odoo/odoo#166566