convert: sometimes git forgets the author

hgext/convert/git.py

@@ -82,6 +82,7 b' class convert_git(converter_source):'
         tzs, tzh, tzm = tz[-5:-4] + "1", tz[-4:-2], tz[-2:]
         tz = -int(tzs) * (int(tzh) * 3600 + int(tzm))
         date = tm + " " + str(tz)
+        author = author or unknown
 
         c = commit(parents=parents, date=date, author=author, desc=message)
         return c