placeholder command for sgn_config

Signal_generator_config command could have been empty under certain circumstances.
This commit is preventing this.

Author: sawenzel

MC/bin/o2dpg_sim_workflow.py

@@ -271,6 +271,7 @@ def getDPL_global_options(bigshm=False,nosmallrate=False):
 
    # produce the signal configuration
    SGN_CONFIG_task=createTask(name='gensgnconf_'+str(tf), tf=tf, cwd=timeframeworkdir)
+   SGN_CONFIG_task['cmd'] = 'echo "placeholder / dummy task"'
    if GENERATOR == 'pythia8':
       SGN_CONFIG_task['cmd'] = '${O2DPG_ROOT}/MC/config/common/pythia8/utils/mkpy8cfg.py \
                                 --output=pythia8.cfg                                     \