code: def test_fixed_none_value_sampling() -> None:
      def objective(trial: Trial) -> float:
          trial.suggest_categorical("x", (None, 0))
          return 0.0
  
      tpe = optuna.samplers.TPESampler()
  
      with warnings.catch_warnings():
          warnings.simplefilter(
              "ignore", optuna.exceptions.ExperimentalWarning
          )
          # In this following case , "x" should sample only `None`
          sampler = optuna.samplers.PartialFixedSampler(
              fixed_params={"x": None}, base_sampler=tpe
          )
  
      study = optuna.create_study(sampler=sampler)
      study.optimize(objective, n_trials=10)
  
      for trial in study.trials:
          assert trial.params["x"] is None
  
file_path: optuna/tests/samplers_tests/test_partial_fixed.py
imports: from unittest.mock import patch
  import warnings
  import pytest
  import optuna
  from optuna.samplers import PartialFixedSampler
  from optuna.samplers import RandomSampler
  from optuna.trial import Trial
label_desc: Use image node_id 7 for calling a global function with example usage: test_fixed_none_value_sampling() and returns: int