code: def to_json(self):
      # prepare json with all parameters
      if len(self._na_fill_params) == 0:
          return {}
      params = {
          "fill_method": self._na_fill_method,
          "fill_params": self._na_fill_params,
          "datetime_columns": list(self._datetime_columns),
      }
      for col in self._datetime_columns:
          params["fill_params"][col] = str(params["fill_params"][col])
      return params
  
file_path: mljar-supervised/supervised/preprocessing/preprocessing_missing.py
imports: import json
  import os
  import numpy
  import pandas
  from supervised.preprocessing.preprocessing_utils import PreprocessingUtils
label_desc: Use image node_id 8 for calling the PreprocessingMissingValues obj's underlying member method code with example usage: obj.to_json() and returns: params, dict