handle null values in logfiles

[anarcat]

In my experience, the CSV logfile frequently gets garbage written to
it. It's unclear to me why that happens: presumably the update script
does that when there's a power outage or suspend or something weird
that frequently happens on laptops.

Instead of trying to figure out how the impossible happens, I figured
it might be better to just deal with it better and silence those
errors. I tried to do this in a better way by using an open hook, but
it's actually quite difficult to do so because compressed handles
don't support customizing the encoding routines the same way io.open
does. Instead we monkeypatch the iterator eventually used by the CSV
parser, which works in my tests.