Do not trigger callbacks if undefined

Change-Id: I21c2aeab910c620d46c0239a36c2ee9f62e7c026
Reviewed-on: http://gerrit.aldebaran.lan/50696
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Tested-by: llec <llec@aldebaran-robotics.com>