@Override
  public ColumnTransformer visitNullOperand(
      NullOperand nullOperand, ColumnTransformerVisitorContext context) {
    ColumnTransformer res =
        context.cache.computeIfAbsent(
            nullOperand,
            e -> {
              NullColumnTransformer columnTransformer = new NullColumnTransformer();
              context.leafList.add(columnTransformer);
              return columnTransformer;
            });
    res.addReferenceCount();
    return res;
  }
Responsible for constructing {@link ColumnTransformer} through Expression.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/visitor/ColumnTransformerVisitor.java