IMPALA-914: Map null type to boolean in JDBC to be compatible with Hive.

Change-Id: I5831ae7d5dcb03aecea4138d0b13487898951068
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2279
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins