private static EnumSet<Feature> enumSet(Collection<Feature> features) {
            return features.isEmpty() ? EnumSet.noneOf(Feature.class) : EnumSet.copyOf(features);
        }
Allows null values to be provided for an optional structure member.

<p>By default, null values are not allowed for optional types.
smithy-model/src/main/java/software/amazon/smithy/model/validation/NodeValidationVisitor.java