Mailmap: Allow empty email addresses to be mapped

Mailmap: Allow empty email addresses to be mapped

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>