A repository that takes commits from people it cannot list has no way
to freeze a range today: a null `authors` falls back to
`git config user.email`, so every foreign author or committer fails.
Null, or no key, now means neither is checked; the freeze warns that it
did not check them and records `authors` as null. A list is checked as
before. An empty list, which no commit could pass, is a config error
that points at null.
Released as 1.1.0.
Jorijn van der Graaf
c6b4d78245