reflog: don't be noisy on empty reflogs

reflog: don't be noisy on empty reflogs

Fix a regression in my daf1d8285ee (reflog expire: don't use
lookup_commit_reference_gently(), 2021-12-22), in changing from
lookup_commit_reference_gently() to lookup_commit() we stopped trying
to call deref_tag() and parse_object() on the provided OID, but we
also started returning non-NULL for the null_oid().

As a result we'd emit an error() via mark_reachable() later in this
function as we tried to invoke parse_commit() on it.

Reported-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Tested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>