code: def fill_matrix_nulls(matrix, null_value):
      """Fills null values in the matrix with a given value.
  
      :param matrix: The matrix to fill.
      :type matrix: list
      :param null_value: The value to fill null values with.
      :type null_value: int
      :returns: The filled matrix.
      :rtype: numpy.ndarray
      """
      idx_arrays = np.where(matrix.isnull())
      idx_tuples = list(zip(idx_arrays[0], idx_arrays[1]))
      for tuple in idx_tuples:
          matrix.iloc[tuple] = null_value
  
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 10 for calling a global function with example usage: fill_matrix_nulls(matrix, null_value) without return types