### 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.
"""Helper to return tasks that would be cleared if **upstream** selected."""
        return set(
            task.dag.partial_subset(
                task_ids_or_regex=[task.task_id],
                include_downstream=False,
                include_upstream=False,
            ).tasks
        )
    def test_get_flat_relative_ids_with_setup(self):
        with DAG(dag_id="test_dag", start_date=pendulum.now()) as dag:
            s1, w1, w2, w3, w4, t1 = self.make_tasks(dag, "s1, w1, w2, w3, w4, t1")
        s1 >> w1 >> w2 >> w3
        
        assert set(w1.get_upstreams_only_setups_and_teardowns()) == {s1}
        
        assert set(w2.get_upstreams_only_setups_and_teardowns()) == {s1}
        assert set(w3.get_upstreams_only_setups_and_teardowns()) == {s1}
        
        assert self.cleared_downstream(w2) == {s1, w2, w3}
        w3 >> t1
        
        assert set(w2.get_upstreams_only_setups_and_teardowns()) == {s1}
        
        
        assert self.cleared_downstream(w2) == {s1, w2, w3, t1}