t5004: avoid using tar for empty packages

t5004: avoid using tar for empty packages

ea2d20d4c2 ("t5004: avoid using tar for checking emptiness of archive",
2013-05-09), introduced a fake empty tar archive to allow for portable
tests of emptiness without having to invoke tar

4318094047 ("archive: don't add empty directories to archives", 2017-09-13)
changed the expected result for its tests from one containing an empty
directory to a plain empty archive but the portable test wasn't updated
resulting on them failing again in (at least) NetBSD and OpenBSD

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>