Setting empty strings to null in DynamoDB

The convertEmptyValues option in the Amazon DynamoDb DocumentClient when set to true, should automatically convert empty strings, blobs, and sets to null.

https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/DynamoDB/Converter.html

For the below code,

const aKey = datetime + " - " + aRandom

const info = {
          attr1 : "derf",
          attr2 : ''
      }


AWS.config.credentials.get(function(){
    ddbdoc_get('newTable',aKey,"shorn",info)
});


function ddbdoc_get(tableName, hashId, sortID , info) {

    var docClient = new AWS.DynamoDB.DocumentClient({ convertEmptyValues : true});


    var params = {
      TableName: tableName,
      Item : { id: hashId,
        name : sortID,
        ...info
       },

    };

   docClient.put(params, function(err, data) {
      if (err) {
        console.log("Error", err);
      } else {
        console.log("Success", data.Item);
      }
    });
  }

In the database, attr3 is being set in dynamoDB database as true and not as null( the item after 'derf')

how to set empty string to null in DynamoDB?