### 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 {
            (dag_id, False),
            (subdag_id, False),
        } == set(unpaused_dags)
        DagModel.get_dagmodel(dag.dag_id).set_is_paused(is_paused=True, including_subdags=False)
        paused_dags = (
            session.query(DagModel.dag_id, DagModel.is_paused)
            .filter(
                DagModel.dag_id.in_([subdag_id, dag_id]),
            )
            .all()
        )
        assert {
            (dag_id, True),
            (subdag_id, False),
        } == set(paused_dags)
        DagModel.get_dagmodel(dag.dag_id).set_is_paused(is_paused=True)
        paused_dags = (
            session.query(DagModel.dag_id, DagModel.is_paused)
            .filter(
                DagModel.dag_id.in_([subdag_id, dag_id]),
            )
            .all()
        )
        assert {
            (dag_id, True),
            (subdag_id, True),
        } == set(paused_dags)
    def test_existing_dag_is_paused_upon_creation(self):