title: DynamoDB mapper: update only not-null properties
body: <p>I am using the mapper provided by the AWS SDK, with Java. 
  I need to update record on DynamoDB: is there any way, using the mapper, to avoid that a null property of a mapped entity is stored on DynamoDB, overwriting the old value? </p>
  
  <p>I try to explain my question with an example.
  I have a java entity with three properties: id, a, b. It is mapped to a DynamoDB table with hashKey only, on field id.
  On DynamoDB it is stored a record {"id":"1", "a":"aa"}.
  After an update called on an entity with id:1, a:null and b:"bb", I find on DynamoDB a record {"id":"1", "b":"bb"}.</p>
  
  <p>Any solutions?</p>
  
  <p>Thanks</p>