fix: Return a list instead of None in RelationalBone.relskels_from_keys

[ArneGudermann]

Fix the Bug that this fails when

viur-core/src/viur/core/bones/relational.py

Lines 1185 to 1192 in 7801292

	rel_list = self.relskels_from_keys(parsed_value)
	if append:
	if language:
	skel[boneName][language].extend(rel_list)
	else:
	if not isinstance(skel[boneName], list):
	skel[boneName] = []
	skel[boneName].extend(rel_list)

this returns None

viur-core/src/viur/core/bones/relational.py

Lines 1102 to 1103 in 7801292

	if not all(db_objs := db.Get([db.keyHelper(value[0], self.kind) for value in key_rel_list])):
	return None

[sveneberth]

LGTM