### 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.
child_dag = DAG(
            "parent_dag.child_dag",
            schedule="@daily",
            start_date=DEFAULT_DATE,
        )
        with child_dag:
            EmptyOperator(task_id="a_child")
            EmptyOperator(task_id="b_child")
        parent_dag = DAG(
            "parent_dag",
            schedule="@daily",
            start_date=DEFAULT_DATE,
        )
        
        with parent_dag:
            op1 = EmptyOperator(task_id="a_parent")
            op2 = SubDagOperator(task_id="child_dag", subdag=child_dag)
            op3 = EmptyOperator(task_id="b_parent")
            op1 >> op2 >> op3
        topological_list = parent_dag.topological_sort(include_subdag_tasks=True)
        assert self._occur_before("a_parent", "child_dag", topological_list)
        assert self._occur_before("child_dag", "a_child", topological_list)
        assert self._occur_before("child_dag", "b_child", topological_list)
        assert self._occur_before("a_child", "b_parent", topological_list)
        assert self._occur_before("b_child", "b_parent", topological_list)
    def test_dag_topological_sort_dag_without_tasks(self):