[FIX] purchase: vendor reference

In case no PO has a vendor reference, keep the value.

addons/purchase/models/account_invoice.py

@@ -103,7 +103,7 @@ class AccountInvoice(models.Model):
         purchase_ids = self.invoice_line_ids.mapped('purchase_id')
         if purchase_ids:
             self.origin = ', '.join(purchase_ids.mapped('name'))
-            self.reference = ', '.join(purchase_ids.filtered('partner_ref').mapped('partner_ref'))
+            self.reference = ', '.join(purchase_ids.filtered('partner_ref').mapped('partner_ref')) or self.reference
 
     @api.onchange('partner_id', 'company_id')
     def _onchange_partner_id(self):