In commit 741421de, we accidently made it so that is_null=true
StringVals became is_null=false with len=0. Fix that and add
a regression test.
Change-Id: I34d288aad66a2609484058c9a177c02200cb6a6e
Reviewed-on: http://gerrit.cloudera.org:8080/7364
Reviewed-by: Bharath Vissapragada <bharathv@cloudera.com>
Tested-by: Impala Public Jenkins