list-objects: check if filter is NULL before using

In partial_clone_get_default_filter_spec(), the
core_partial_clone_filter_default variable may be NULL; ensure that it
is not NULL before using it.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects-filter-options.c

@@ -146,6 +146,8 @@ void partial_clone_get_default_filter_spec(
 	/*
 	 * Parse default value, but silently ignore it if it is invalid.
 	 */
+	if (!core_partial_clone_filter_default)
+		return;
 	gently_parse_list_objects_filter(filter_options,
 					 core_partial_clone_filter_default,
 					 NULL);

t/t0410-partial-clone.sh

@@ -23,7 +23,15 @@ promise_and_delete () {
 	delete_object repo "$HASH"
 }
 
+test_expect_success 'extensions.partialclone without filter' '
+	test_create_repo server &&
+	git clone --filter="blob:none" "file://$(pwd)/server" client &&
+	git -C client config --unset core.partialclonefilter &&
+	git -C client fetch origin
+'
+
 test_expect_success 'missing reflog object, but promised by a commit, passes fsck' '
+	rm -rf repo &&
 	test_create_repo repo &&
 	test_commit -C repo my_commit &&