[FIX] point_of_sale: close sessions via xml-rpc

Some of the support scripts use xml-rpc calls to work
on pos.session on Saas or SH databases
(especially when needed to upload lot of offline orders
in smaller batches)
These scripts should be allowed to close any rescue session
created during this process programmatically via xml-rpc.

Our xml-rpc protocol does not allow to call function that
return nothing

closes odoo/odoo#61988

Signed-off-by: Richard Mathot (rim) <rim@openerp.com>

addons/point_of_sale/models/pos_session.py

@@ -259,6 +259,7 @@ class PosSession(models.Model):
             session.write({'state': 'closing_control', 'stop_at': fields.Datetime.now()})
             if not session.config_id.cash_control:
                 session.action_pos_session_close()
+        return True
 
     @api.multi
     def _check_pos_session_balance(self):
@@ -271,6 +272,7 @@ class PosSession(models.Model):
     def action_pos_session_validate(self):
         self._check_pos_session_balance()
         self.action_pos_session_close()
+        return True
 
     @api.multi
     def action_pos_session_close(self):