Null simulation group crashes the submit_group script

[Baguage]

Transferred from redmine, ticket #6213

When the "simulation_group" key is an execution request is "null", the submit_group script crashes. The script assumes that the group will have a member called "simulations". So this exception is raised:

File ".../submit_group.py", line 76, in main for simulation in execution_request.simulation_group.simulations: AttributeError: 'NoneType' object has no attribute 'simulations'