I think "<unknown>" is a better start point.
Both committer and author are defaulted to "<unknown>" in rev 2c207d528e37e59d8d8d14e24edd14fb34a10a21.
I'm using the value from the author cert as the git author and the key from the same cert as the git committer. That should generally be the same key that would be on a changelog cert but it's not guaranteed to be.
I've also removed the code that was trying to clean up various things, like adding < and > around email addresses. These should all be fixed by mappings using the --authors-file now.
The list of committers that might need mapping comes from:
$ mtn db execute 'select distinct keypair from revision_certs where name = "author"'
The list of authors that might need mapping comes from:
$ mtn db execute 'select distinct value from revision_certs where name = "author"'