### 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.
with DAG(dag_id="test_dag", start_date=pendulum.now()) as dag:
            s1, w1, w2, t1 = self.make_tasks(dag, "s1, w1, w2, t1")
        s1 >> w1 >> [w2, t1]
        s1 >> t1
        
        assert set(w2.get_upstreams_only_setups_and_teardowns()) == set()
        
        assert set(w1.get_upstreams_only_setups_and_teardowns()) == {s1, t1}
        
        assert self.cleared_downstream(w1) == {s1, w1, w2, t1}
        assert self.cleared_downstream(w2) == {w2}
        
        s2 = BaseOperator(task_id="s2", dag=dag).as_setup()
        t1 >> s2
        assert w1.get_flat_relative_ids(upstream=False) == {"t1", "w2", "s2"}
        assert self.cleared_downstream(w1) == {s1, w1, w2, t1, s2}
    def test_get_flat_relative_ids_two_tasks_diff_setup_teardowns(self):
        with DAG(dag_id="test_dag", start_date=pendulum.now()) as dag:
            s1, t1, s2, t2, w1, w2 = self.make_tasks(dag, "s1, t1, s2, t2, w1, w2")
        s1 >> w1 >> [w2, t1]
        s1 >> t1
        s2 >> t2
        s2 >> w2 >> t2
        assert set(w1.get_upstreams_only_setups_and_teardowns()) == {s1, t1}
        
        assert self.cleared_downstream(w1) == {s1, w1, t1, s2, w2, t2}
        assert self.cleared_neither(w1) == {s1, w1, t1}
        assert set(w2.get_upstreams_only_setups_and_teardowns()) == {s2, t2}
        assert self.cleared_downstream(w2) == {s2, w2, t2}
    def test_get_flat_relative_ids_one_task_multiple_setup_teardowns(self):