### 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.
cls._context_managed_dags.appendleft(dag)
    @classmethod
    def pop_context_managed_dag(cls) -> DAG | None:
        dag = cls._context_managed_dags.popleft()
        
        if cls.current_autoregister_module_name is not None and dag and dag.auto_register:
            mod = sys.modules[cls.current_autoregister_module_name]
            cls.autoregistered_dags.add((dag, mod))
        return dag
    @classmethod
    def get_current_dag(cls) -> DAG | None:
        try:
            return cls._context_managed_dags[0]
        except IndexError:
            return None
def _run_trigger(trigger):
    async def _run_trigger_main():
        async for event in trigger.run():
            return event
    return asyncio.run(_run_trigger_main())
def _run_task(*, ti: TaskInstance, inline_trigger: bool = False, session: Session):