[FIX] purchase: default user_id on bill

- Create a PO, change the Purchase Representative (`user_id`)
- Create the corresponding vendor bill

The `user_id` of the PO is not transferred to the invoice.

There is no reason to not keep the value.

opw-2046963

closes odoo/odoo#35611

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/purchase/models/purchase.py

@@ -404,6 +404,8 @@ class PurchaseOrder(models.Model):
             'default_company_id': self.company_id.id,
             'company_id': self.company_id.id
         }
+        if self.user_id:
+            result['context']['default_user_id'] = self.user_id.id
         # choose the view_mode accordingly
         if len(self.invoice_ids) > 1 and not create_bill:
             result['domain'] = "[('id', 'in', " + str(self.invoice_ids.ids) + ")]"