Bug 581433 - [Robotics] Error during launch script generation of system assemblies executing behavior trees

- Fixed createLaunchScriptSingle() to access its input attribute "part" only when input attribute "addSequencer" is false
- Fixed the definition of empty user parameter for the system sequencer component

Change-Id: I1946995a0f942cee610fdebbad150e96f1f8fbd7
Signed-off-by: Matteo MORELLI <matteo.morelli@cea.fr>