reflog: don't be noisy on empty reflogs

Fix a regression in my daf1d8285e (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>

builtin/reflog.c

@@ -382,6 +382,8 @@ static void reflog_expiry_prepare(const char *refname,
 		cb->unreachable_expire_kind = UE_HEAD;
 	} else {
 		commit = lookup_commit(the_repository, oid);
+		if (commit && is_null_oid(&commit->object.oid))
+			commit = NULL;
 		cb->unreachable_expire_kind = commit ? UE_NORMAL : UE_ALWAYS;
 	}
 

t/t1410-reflog.sh

@@ -422,4 +422,13 @@ test_expect_success 'expire with multiple worktrees' '
 	)
 '
 
+test_expect_success REFFILES 'empty reflog' '
+	test_when_finished "rm -rf empty" &&
+	git init empty &&
+	test_commit -C empty A &&
+	>empty/.git/logs/refs/heads/foo &&
+	git -C empty reflog expire --all 2>err &&
+	test_must_be_empty err
+'
+
 test_done