t5562: do not depend on /dev/zero

t5562: do not depend on /dev/zero

It was reported [1] that NonStop platform does not have /dev/zero.

The test uses /dev/zero as a dummy input. Passing case (http-backed
failed because of too big input size) should not be reading anything
from it. If http-backend would erroneously try to read any data
returning EOF probably would be even safer than providing some
meaningless data.

Replace /dev/zero with /dev/null to avoid issues with platforms which do
not have /dev/zero.

[1] https://public-inbox.org/git/20190209185930.5256-4-randall.s.becker@rogers.com/

Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>