Fix CompoundPredicate::CodegenComputeFn

The codegen'd function was returning false instead of NULL

Change-Id: I481b5b82e3336a71c2a55a7be326af8d39a4d510
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4394
Tested-by: jenkins
Reviewed-by: Nong Li <nong@cloudera.com>