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
```
issue