If no authorities votes on a BadExit flag, doctor crashes

Parse consensuses without any BadExit votes. Doctor crashes with the following error

Traceback (most recent call last):
  File "./consensus_health_checker.py", line 744, in <module>
    main()
  File "./consensus_health_checker.py", line 243, in main
    issues += run_checks(consensuses, votes)
  File "./consensus_health_checker.py", line 325, in run_checks
    issues = checker(latest_consensus, consensuses, votes)
  File "./consensus_health_checker.py", line 649, in bad_exits_in_sync
    agreed_bad_exits = set.intersection(*bad_exits.values())
TypeError: descriptor 'intersection' of 'set' object needs an argument