### 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.
assert op1.dag is dag
        assert op1.owner == "owner1"
        assert op2.dag is dag2
        assert op2.owner == "owner2"
        with dag2:
            op3 = EmptyOperator(task_id="op3")
        assert op3.dag is dag2
        assert op3.owner == "owner2"
        with dag:
            with dag2:
                op4 = EmptyOperator(task_id="op4")
            op5 = EmptyOperator(task_id="op5")
        assert op4.dag is dag2
        assert op5.dag is dag
        assert op4.owner == "owner2"
        assert op5.owner == "owner1"
        with DAG("creating_dag_in_cm", start_date=DEFAULT_DATE) as dag:
            EmptyOperator(task_id="op6")
        assert dag.dag_id == "creating_dag_in_cm"
        assert dag.tasks[0].task_id == "op6"
        with dag:
            with dag:
                op7 = EmptyOperator(task_id="op7")
            op8 = EmptyOperator(task_id="op8")
        op9 = EmptyOperator(task_id="op8")
        op9.dag = dag2
        assert op7.dag == dag
        assert op8.dag == dag
        assert op9.dag == dag2
    def test_dag_topological_sort_include_subdag_tasks(self):