Code review comment for ~txiao/charm-logrotated:handle_unset

Good catch. It's shame that we didn't aware this in the previous code review
https://code.launchpad.net/~llama-charmers/charm-logrotated/+git/charm-logrotated/+merge/435334

Furthermore, we should not skip Diko's suggest in the MR too. From the source code, I do not find the code to reset the value. ( set -> unset, or random -> unset ). This can be another feature request. I will let Kamal review the code and confirm my idea is correct.