if no bridges in the network-status have Running flag, that doesn't mean they're all down

In tpo/anti-censorship/rdsys#102 (closed) we discovered a fun surprise: since Serge, the bridge authority, uses the same approach to generate networkstatus documents as the v3 dir auths use, then if Serge is generating a networkstatus document within 30 minutes (TestingAuthDirTimeToLearnReachability) of starting up, it will not express an opinion about the Running flag for each bridge it lists.

The fix we're putting in for rdsys is: if no bridge in the networkstatus list has the Running flag, then what it means instead is: we don't have any new info about whether those bridges are Running, and we should do something smart when interpreting the data, like use the most recent info we do have.

I've opened this ticket in the onionoo component, but if there are other pieces of metrics that need to get smarter behavior here, please open tickets for them too. :)