code: protected Map<String, AttributeValue> transformAttributes(String hashKey, String rangeKey,
              Map<String, AttributeValue> attributeValues) {
          return attributeValues;
      }
docs: Transform attribute values prior to storing in DynamoDB.
  Only ever called by {@link #transformAttributes(Class, Map)}.
  By default, returns the attributes unchanged.
  
  @param hashKey the attribute name of the hash key
  @param rangeKey the attribute name of the range key (or null if there is none)
  @param attributeValues
  @return the encrypted attributes