Method has_open_run

cpp/src/arrow/array/builder_run_end.h:150–150

  std::shared_ptr<DataType> type() const override { return inner_builder_->type(); }

  bool has_open_run() const { return current_run_length_ > 0; }
  int64_t open_run_length() const { return current_run_length_; }

 private: