whodunnit consistency

[PaperTrail `whodunnit` column value](https://github.com/paper-trail-gem/paper_trail#4a-finding-out-who-was-responsible-for-a-change) is currently copied over to the FV `whodunnit_id` and parsed to integer. The `String` value edge-case is not handled properly and this results with a `0` value of the `whodunnit_id`