When trying to import from svn using an author file, git-svn bails out
if it encounters a blank author. The attached patch changes this
behavior and allow using the author file with blanks authors.

I came across this bug while importing from a cvs2svn repo where the
initial revision (1) has a blank author. This doesn't break the behavior
of bailing out when an unknown author is encountered.
Acked-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>