Avoid aggregate.py error when no nodes are measured

If the scanner cron runs early, errors like this can happen:

Traceback (most recent call last):
  File "/Users/bwauth/torflow/NetworkScanners/BwAuthority/aggregate.py", line 894, in <module>
    main(sys.argv)
  File "/Users/bwauth/torflow/NetworkScanners/BwAuthority/aggregate.py", line 798, in main
    nodes.itervalues())))
ValueError: min() arg is an empty sequence