[FIX] purchase: keep vendor reference

Create a PO and assign any string to the Vendor Reference field. Confirm
and create an invoice.

The Vendor Reference field on the invoice is empty, while it is expected
to be filled in.

opw-1879379

addons/purchase/models/account_invoice.py

@@ -101,8 +101,8 @@ class AccountInvoice(models.Model):
     @api.onchange('invoice_line_ids')
     def _onchange_origin(self):
         purchase_ids = self.invoice_line_ids.mapped('purchase_id')
-        if purchase_ids:
-            self.origin = ', '.join(purchase_ids.mapped('name'))
+        self.origin = ', '.join(purchase_ids.mapped('name'))
+        self.reference = ', '.join(purchase_ids.filtered('partner_ref').mapped('partner_ref'))
 
     @api.onchange('partner_id', 'company_id')
     def _onchange_partner_id(self):