[FrameworkBundle] Fixed OutOfBoundException when session handler_id is null

When a null is provided for framework.session.handler_id the FrameworkExtension attempts to set the session storage to null for the 'session.storage.php_bridge' by altering the second argument. According to the session.xml service definition, there is no second argument, and it is in fact the first (read, 0 index) argument that should be changed.