public final AttributeValue convert(T o) {
                    return o == null ? new AttributeValue().withNULL(true) : super.convert(o);
                }
Gets the nested converter for the given conversion type.
Also wraps the resulting converter with a nullable converter.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/StandardModelFactories.java