@Override
            boolean isStructureMemberOptional(StructureShape container, MemberShape member, Shape target) {
                if (target instanceof StructureShape || target instanceof UnionShape) {
                    return true;
                }

                return CLIENT.isStructureMemberOptional(container, member, target);
            }
Like {@link #CLIENT} mode, but will treat all members that target
structures and unions as optional because these members can never
transition to optional using a default trait.
smithy-model/src/main/java/software/amazon/smithy/model/knowledge/NullableIndex.java