### Body

Currently `start_date` is mandatory parameter.
For DAGs with `schedule=None` we can relax this requirement as no scheduling calculation needed so the `start_date` parameter isn't used.

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
)
        task_instance_1 = TI(t_1, execution_date=DEFAULT_DATE, state=State.RUNNING)
        task_instance_2 = TI(t_2, execution_date=DEFAULT_DATE, state=State.RUNNING)
        session.merge(task_instance_1)
        session.merge(task_instance_2)
        return dag, subdag
    @pytest.mark.parametrize("dag_run_state", [DagRunState.QUEUED, DagRunState.RUNNING])
    def test_clear_set_dagrun_state_for_subdag(self, dag_run_state):
        session = settings.Session()
        dag, subdag = self._make_test_subdag(session)
        session.flush()
        dag.clear(
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=1),
            dag_run_state=dag_run_state,
            include_subdags=True,
            include_parentdag=False,
            session=session,
        )
        dagrun = (
            session.query(
                DagRun,
            )
            .filter(DagRun.dag_id == subdag.dag_id)
            .one()
        )
        assert dagrun.state == dag_run_state
        session.rollback()
    @pytest.mark.parametrize("dag_run_state", [DagRunState.QUEUED, DagRunState.RUNNING])
    def test_clear_set_dagrun_state_for_parent_dag(self, dag_run_state):