Modified server response model to allow for null images

* Changed serialization to check if image field is null or empty before
  attempting serialization
* Added unit tests to verify serialization does not fail for the server
  response object if the image field is the empty string

Change-Id: Ifc9df4d5288e64a173ad2c5c882469ae75dcad5b
Merge "Modified server response model to allow for null images"