Function AppendNull

cpp/src/arrow/array/builder_union.h:109–117

        offsets_builder_(pool, alignment) {}

  Status AppendNull() final {
    const int8_t first_child_code = type_codes_[0];
    ArrayBuilder* child_builder = type_id_to_children_[first_child_code];
    ARROW_RETURN_NOT_OK(types_builder_.Append(first_child_code));
    ARROW_RETURN_NOT_OK(
        offsets_builder_.Append(static_cast<int32_t>(child_builder->length())));
    // Append a null arbitrarily to the first child
    return child_builder->AppendNull();
  }

  Status AppendNulls(int64_t length) final {
    const int8_t first_child_code = type_codes_[0];