code: def test_default_datetime(session: Session) -> None:
      # Regardless of the initial state the trial created here should have null datetime_start
      session.add(TrialModel(state=TrialState.WAITING))
      session.commit()
  
      trial_model = session.query(TrialModel).first()
  
      assert trial_model is not None
      assert trial_model.datetime_start is None
      assert trial_model.datetime_complete is None
  
imports: from datetime import datetime
  import pytest
  from sqlalchemy import create_engine
  from sqlalchemy.exc import IntegrityError
  from sqlalchemy.orm import Session
  from optuna.storages._rdb.models import BaseModel
  from optuna.storages._rdb.models import StudyDirectionModel
  from optuna.storages._rdb.models import StudyModel
  from optuna.storages._rdb.models import StudySystemAttributeModel
  from optuna.storages._rdb.models import TrialHeartbeatModel
  from optuna.storages._rdb.models import TrialIntermediateValueModel
  from optuna.storages._rdb.models import TrialModel
  from optuna.storages._rdb.models import TrialSystemAttributeModel
  from optuna.storages._rdb.models import TrialUserAttributeModel
  from optuna.storages._rdb.models import TrialValueModel
  from optuna.storages._rdb.models import VersionInfoModel
  from optuna.study._study_direction import StudyDirection
  from optuna.trial import TrialState
label_desc: Use image node_id 1 for calling the TestTrialModel obj's underlying member method code with example usage: obj.test_default_datetime(session) without return types