update serialization to skip empty values

Ranger plugins download policies and tags from Ranger admin via REST API calls. Ranger admin serializes policies and tags to Json format in response to requests from plugins. During serialization, attributes having null values are skipped to avoid unnecessary data in the response, thereby reducing the size of the response. In addition to skipping null values, it will help to skip empty values as well. This can reduce the response size further by about 20%.