code: def test_empty_cohort_list_serialization(self):
      json_str = json.dumps(None, default=cohort_filter_json_converter)
      assert "null" in json_str
  
imports: import json
  import pandas
  import pytest
  from raiutils.cohort import ClassificationOutcomes, Cohort, CohortFilter, CohortFilterMethods, cohort_filter_json_converter
  from raiutils.exceptions import UserConfigValidationException
label_desc: Use image node_id 2 for calling the TestCohortList obj's underlying member method code with example usage: obj.test_empty_cohort_list_serialization() without return types