preDag divides by 0 when all probe jobs fails

[belforte]

ref: https://hypernews.cern.ch/HyperNews/CMS/get/computing-tools/6264/1.html
and related to dmwm/CRABClient#5142

when all probe jobs fail PreDag should detect it and handle properly, not blindly raise exception in
computing events throughput in

CRABServer/src/python/TaskWorker/Actions/PreDAG.py

Lines 209 to 210 in 67a7bd6

	eventsThr = sumEventsThr / count
	eventsSize = sumEventsSize / count

Tue, 21 Dec 2021 00:24:12 CET(+0000):INFO:PreDAG Pre-DAG started with output redirected to /data/srv/glidecondor/condor_local/spool/6577/0/cluster75266577.proc0.subproc0/prejob_logs/predag.0.txt
Tue, 21 Dec 2021 00:24:12 CET(+0000):INFO:PreDAG found 5 completed jobs
Tue, 21 Dec 2021 00:24:12 CET(+0000):INFO:PreDAG jobs remaining to process: 0-1, 0-2, 0-3, 0-4, 0-5
Tue, 21 Dec 2021 00:24:12 CET(+0000):INFO:PreDAG jobs remaining to process: 0-1, 0-2, 0-3, 0-4, 0-5
Got a fatal exception: division by zero
Traceback (most recent call last):
  File "/usr/lib64/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib64/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/data/srv/glidecondor/condor_local/spool/6577/0/cluster75266577.proc0.subproc0/TaskWorker/TaskManagerBootstrap.py", line 85, in <module>
    retval = bootstrap()
  File "/data/srv/glidecondor/condor_local/spool/6577/0/cluster75266577.proc0.subproc0/TaskWorker/TaskManagerBootstrap.py", line 28, in bootstrap
    return PreDAG.PreDAG().execute(*sys.argv[2:])
  File "TaskWorker/Actions/PreDAG.py", line 114, in execute
    retval = self.executeInternal(*args)
  File "TaskWorker/Actions/PreDAG.py", line 209, in executeInternal
    eventsThr = sumEventsThr / count
ZeroDivisionError: division by zero

[belforte]

closed via #6959 in python3 branch