[FIX] purchase: overwrite vendor reference

Overwrite vendor reference only if there is a PO linked to the bill.
Otherwise, any addition of product will erase what the user just typed.

Closes #26734

addons/purchase/models/account_invoice.py

@@ -101,8 +101,9 @@ class AccountInvoice(models.Model):
     @api.onchange('invoice_line_ids')
     def _onchange_origin(self):
         purchase_ids = self.invoice_line_ids.mapped('purchase_id')
-        self.origin = ', '.join(purchase_ids.mapped('name'))
-        self.reference = ', '.join(purchase_ids.filtered('partner_ref').mapped('partner_ref'))
+        if purchase_ids:
+            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):