### 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.
local_tz = pendulum.timezone("Europe/Zurich")
        default_args = {"start_date": datetime.datetime(2018, 1, 1, tzinfo=local_tz)}
        dag = DAG("DAG", default_args=default_args)
        assert dag.timezone.name == local_tz.name
        dag = DAG("DAG", default_args=default_args)
        assert dag.timezone.name == local_tz.name
    def test_roots(self):
        """Verify if dag.roots returns the root tasks of a DAG."""
        with DAG("test_dag", start_date=DEFAULT_DATE) as dag:
            op1 = EmptyOperator(task_id="t1")
            op2 = EmptyOperator(task_id="t2")
            op3 = EmptyOperator(task_id="t3")
            op4 = EmptyOperator(task_id="t4")
            op5 = EmptyOperator(task_id="t5")
            [op1, op2] >> op3 >> [op4, op5]
            assert set(dag.roots) == {op1, op2}
    def test_leaves(self):
        """Verify if dag.leaves returns the leaf tasks of a DAG."""
        with DAG("test_dag", start_date=DEFAULT_DATE) as dag:
            op1 = EmptyOperator(task_id="t1")
            op2 = EmptyOperator(task_id="t2")
            op3 = EmptyOperator(task_id="t3")
            op4 = EmptyOperator(task_id="t4")
            op5 = EmptyOperator(task_id="t5")
            [op1, op2] >> op3 >> [op4, op5]
            assert set(dag.leaves) == {op4, op5}
    def test_tree_view(self):