Improve ThriftConvert handling of nulls and variables in round tripping

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1624022 13f79535-47bb-0310-9956-ffa450edef68
Further tests for ThriftConvert

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1624023 13f79535-47bb-0310-9956-ffa450edef68