CollecTor directory listing apparently breaks when a directory in recent/ runs dry

The CollecTor directory listings on Tor Metrics broke at or shortly after 2017-12-11 13:08:

https://metrics.torproject.org/collector/recent/relay-descriptors/server-descriptors/

The index.json file looks okay right now:

https://collector.torproject.org/index/index.json

I suspect the reason is that some of the directories in recent/ containing sanitized bridge descriptors ran dry yesterday afternoon. Maybe we ran into a NullPointerException or something.