shortlog: don't warn on empty author

Git tries to avoid creating a commit with an empty author name or email. However, commits created by older, less strict versions of git may still be in the history. There's not much point in issuing a warning to stderr for an empty author. The user can't do anything about it now, and we are better off to simply include it in the shortlog output as an empty name/email, and let the caller process it however they see fit. Older versions of shortlog differentiated between "author header not present" (which complained) and "author name/email are blank" (which included the empty ident in the output). But since switching to format_commit_message, we complain to stderr about either case (linux.git has a blank author deep in its history which triggers this). We could try to restore the older behavior (complaining only about the missing header), but in retrospect, there's not much point in differentiating these cases. A missing author header is bogus, but as for the "blank" case, the only useful behavior is to add it to the "empty name" collection. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>