Comment 0 for bug 1812954

If you upgrade from a controller before prune-txn-sleep-time was added as controller-config, to one that uses it (2.4.8-not-released, 2.5.0), then when the Pruner wakes up, it gets a panic() trying to read a value that isn't there.
The value was meant to be optional and fall-back-to-default.

https://github.com/juju/juju/pull/9657