modernize t9300: use test_must_be_empty
modernize t9300: use test_must_be_empty

Instead of comparing actual output to an empty file, use
test_must_be_empty. In addition to the better error message provided by
the helper, allocation of an empty file during the setup sequence can be
avoided.

Signed-off-by: Johannes Sixt <fd432345609ae4d58c1aa629cb66ba96d1391622@kdbg.org>
Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git