### 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.
dag = DAG("dag_paused")
        dag.sync_to_db()
        assert not dag.get_is_paused()
        dag = DAG("dag_paused", is_paused_upon_creation=True)
        dag.sync_to_db()
        
        assert not dag.get_is_paused()
    def test_new_dag_is_paused_upon_creation(self):
        dag = DAG("new_nonexisting_dag", is_paused_upon_creation=True)
        session = settings.Session()
        dag.sync_to_db(session=session)
        orm_dag = session.query(DagModel).filter(DagModel.dag_id == "new_nonexisting_dag").one()
        
        assert orm_dag.is_paused
        session.close()
    def test_existing_dag_default_view(self):