code: def test_env_none_values():
      env = {
          "DASK_A": "None",
          "DASK_B": "NONE",
          "DASK_C": "none",
          "DASK_D": "Null",
          "DASK_E": "NULL",
          "DASK_F": "null",
      }
  
      expected = {
          "a": None,
          "b": None,
          "c": None,
          "d": None,
          "e": None,
          "f": None,
      }
  
      res = collect_env(env)
      assert res == expected
  
imports: from __future__ import annotations
  import os
  import site
  import stat
  import sys
  from collections import OrderedDict
  from contextlib import contextmanager
  import pytest
  import yaml
  import dask.config
  from dask.config import _get_paths, canonical_name, collect, collect_env, collect_yaml, config, deserialize, ensure_file, expand_environment_variables, get, merge, pop, refresh, rename, serialize, update, update_defaults
  from dask.utils import tmpfile
label_desc: Use image node_id 15 for calling a global function with example usage: test_env_none_values() without return types