[FIX] delivery: return label compute field incomplete

Since new ORM in Odoo, the compute methods should always return values.

In the case of return label in delivery, nothing was returned when there
was no carrier on picking.

So we are now setting the field to False when there are no carrier on
picking.

ISSUE-43270

closes odoo/odoo#43386

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>

addons/delivery/models/stock_picking.py

@@ -106,6 +106,8 @@ class StockPicking(models.Model):
         for picking in self:
             if picking.carrier_id:
                 picking.return_label_ids = self.env['ir.attachment'].search([('res_model', '=', 'stock.picking'), ('res_id', '=', picking.id), ('name', 'like', '%s%%' % picking.carrier_id.get_return_label_prefix())])
+            else:
+                picking.return_label_ids = False
 
     @api.depends('move_lines')
     def _cal_weight(self):