IMPALA-1528: TupleIsNullPredicate is never constant

We were treating it as constant before since it has no children and we
didn't override Expr::IsConstant(). However, it's not constant since
it depends on the input tuple, which caused it to blow up when we
tried to evaluate it as a constant expr.

Change-Id: Ic2c3489ba605f03a7644e6ac9107d4310dd0aa7b
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5399
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins