harden _cancel_prolonged_job against missing running_start_time

[soxofaan]

  File ".../openeo/extra/job_management.py", line 474, in run_jobs
   self._job_update_loop(job_db=job_db, start_job=start_job, stats=stats)
  File ".../openeo/extra/job_management.py", line 493, in _job_update_loop
   self._track_statuses(job_db, stats=stats)
  File ".../openeo/extra/job_management.py", line 702, in _track_statuses
   self._cancel_prolonged_job(the_job, active.loc[i])
  File ".../openeo/extra/job_management.py", line 632, in _cancel_prolonged_job
   job_running_start_time = rfc3339.parse_datetime(row["running_start_time"], with_timezone=True)
  File ".../openeo/util.py", line 157, in parse_datetime
   raise ValueError(x)
ValueError: nan

_cancel_prolonged_job should not fail that ugly when running_start_time is not available (various reasons for that)

[HansVRP]

Would it be best to write some "try, except pass" around it?

[soxofaan]

indeed, but I would at least put a warning/info log in the except to make it a bit visible that something did not work as expected