While it makes no sense to map some email address to an empty one, doing
things the other way around can be useful. For example when using
filter-branch with an env-filter that employs a mailmap to fix up an
import that created such broken commits with empty email addresses.
Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>