class_docstr: Base class for aggregation functions.
class_docstr_tok: ["Base","class","for","aggregation","functions","."]
docstr: Specifies what value should be used to replace the missing value.
  
  :returns: The replacement value for the missing value.
  :rtype: any
docstr_tok: ["Specifies","what","value","should","be","used","to","replace","the","missing","value",".",":","returns",":","The","replacement","value","for","the","missing","value",".",":","rtype",":","any"]
code: def _fill_na_value(self):
      """Specifies what value should be used to replace the missing value.
  
      :returns: The replacement value for the missing value.
      :rtype: any
      """
      pass
  
imports: import logging
  import math
  import warnings
  from abc import ABC, abstractmethod
  import numpy
  import pandas
  from sklearn.metrics import multilabel_confusion_matrix
  from erroranalysis._internal.cohort_filter import filter_from_cohort
  from erroranalysis._internal.constants import DIFF, PRED_Y, ROW_INDEX, TRUE_Y, MatrixParams, MetricKeys, Metrics, ModelTask, metric_to_display_name
  from erroranalysis._internal.metrics import get_ordered_classes, is_multi_agg_metric, metric_to_func
  from raiutils.exceptions import UserConfigValidationException
label_desc: Use image node_id 4 for calling the _BaseAggFunc obj's underlying member method code with example usage: obj._fill_na_value() without return types