title: Optional attributes in aws Dynamodbmapper
body: <p>Using Dynamodb mapper how do optional attributes get marshaled? I have a number attribute that only exists for some items, and 0 is a valid value for this attribute. 
  I am observing that for items where this attribute doesn't exist, it's being marshalled as a "0". Any idea how I can differentiate Items where I have a valid value of 0 vs. where the attribute doesn't exist?</p>