We know diff_delta() returns NULL, saying "no good delta exists for
it", when fed an empty data. Check the length of the data in the
caller to avoid such a call.
This incidentally reduces the number of attempted deltification we
see in the final statistics.
Signed-off-by: Mike Hommey <mh@glandium.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>