@Override
  public Double visitLiteral(RexLiteral literal) {
    if (literal.isAlwaysFalse() || RexUtil.isNull(literal)) {
      return 0.0;
    } else if (literal.isAlwaysTrue()) {
      return 1.0;
    } else {
      assert false;
    }
    return null;
  }
Given a RexCall & TableScan find max no of nulls. Currently it picks the
col with max no of nulls.

TODO: improve this

@param call
@param t
@return
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/stats/FilterSelectivityEstimator.java