While not ideal, you can create a new AttributeTransform that sets a place holder when storing into DynamoDB and removes it when pulling out of DynamoDB as part of your DynamoDBMapper instantiation.
I did this to convert some String Sets (SS) in my database to String Lists (L). I almost did this same thing to fix the empty String issue but didn't have the time to implement it yet.
Unfortunately I'm working in a node environment. The Java SDK for working with DynamoDB seems much nicer to work with. For example I believe that you can do transactions with the DynamoDB interface for Java.
Sorry I saw the link was referencing the Java SDK so I thought you were using the same. The DynamoDBMapper Java SDK has been an easy to use and readable ORM for me. Adding annotations to define keys and attributes has been great. That said I'm not sure if JavaScript has an equivalent to the AttributeTransformer I mentioned.
I did this to convert some String Sets (SS) in my database to String Lists (L). I almost did this same thing to fix the empty String issue but didn't have the time to implement it yet.
http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/ama...