docstr: test None comparison queries
docstr_tok: ["test","None","comparison","queries"]
code: def test_null(self):
      """test None comparison queries"""
      msg_ids = self.load_records(10)
  
      query = {"msg_id": None}
      recs = self.db.find_records(query)
      assert len(recs) == 0
  
      query = {"msg_id": {"$ne": None}}
      recs = self.db.find_records(query)
      assert len(recs) >= 10
  
imports: import logging
  import os
  import tempfile
  import time
  from datetime import datetime, timedelta
  from unittest import TestCase
  import pytest
  from jupyter_client.session import Session
  from ipyparallel import util
  from ipyparallel.controller.dictdb import DictDB
  from ipyparallel.controller.hub import init_record
  from ipyparallel.controller.sqlitedb import SQLiteDB
  from ipyparallel.util import utc
label_desc: Use image node_id 15 for calling the TaskDBTest obj's underlying member method code with example usage: obj.test_null() without return types