Update HelperController.php

First : this PR fix a bug on validation in setObjectFieldValueAction if property has no validation constraint setted which throw a 500 error in dev environnement (a notice thrown by RecursiveValidator).

Second : Adding capability to handle inline editing of relation field (many2one relation)
You need to use the related model manager instance (from the admin instance) to retrieve the entity. The ``SonataAdminBundle`` is abtracted.