fail on empty (0-byte) cache files

[boegel]

Traceback (most recent call last):
  File "/usr/bin/release_jobholds", line 186, in <module>
    main()
  File "/usr/bin/release_jobholds", line 182, in main
    opts.epilogue("Release jobholds finished", stats)
  File "/usr/lib/python2.6/site-packages/vsc/utils/script_tools.py", line 162, in epilogue
    self.nagios_reporter._eval_and_exit(**nagios_thresholds)
  File "/usr/lib/python2.6/site-packages/vsc/utils/nagios.py", line 365, in _eval_and_exit
    self.ok(self)
  File "/usr/lib/python2.6/site-packages/vsc/utils/nagios.py", line 332, in ok
    self._exit(NAGIOS_EXIT_OK, msg)
  File "/usr/lib/python2.6/site-packages/vsc/utils/nagios.py", line 329, in _exit
    self._final(nagios_exitcode, msg)
  File "/usr/lib/python2.6/site-packages/vsc/utils/nagios.py", line 171, in cache
    self.log.raiseException("Cannot save to the nagios gzipped JSON file (%s)" % (self.filename))
  File "/usr/lib/python2.6/site-packages/vsc/utils/fancylogger.py", line 216, in raiseException
    raise exception(message)
ValueError: Cannot save to the nagios gzipped JSON file (/var/cache/release_jobholds.nagios.json.gz) (No JSON object could be decoded)