Some CollecTor bridge pool assignment files are empty
I was helping a friend debug some weird bridge churn results for a research project, and noticed that some of CollecTor's bridge pool assignment data files are empty. See for example the file for the timestamp
2021-06-11-19-30-05 retrievable from https://collector.torproject.org/archive/bridge-pool-assignments/bridge-pool-assignments-2021-06.tar.xz.
I'm not sure if this is a bug on the BridgeDB side or the CollecTor side, but it appears to happen regularly (about once a month, near the middle of the month). So we should do an initial investigation on our end and then contact the metrics team if we think this is a CollecTor bug.