Kudu scanner : Expensive per row per column IsNull check

Status KuduScanner::KuduRowToImpalaTuple(const KuduScanBatch::RowPtr& row,
    RowBatch* row_batch, Tuple* tuple) {
  for (int i = 0; i < scan_node_->tuple_desc_->slots().size(); ++i) {
    const SlotDescriptor* info = scan_node_->tuple_desc_->slots()[i];
    void* slot = tuple->GetSlot(info->tuple_offset());

    if (row.IsNull(i)) {
      SetSlotToNull(tuple, *info);
      continue;
    }

    int max_len = -1;
    switch (info->type().type) {
      case TYPE_VARCHAR:
        max_len = info->type().len;
        DCHECK_GT(max_len, 0);