### 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.
external_tasks = session.scalars(query.where(TI.operator == ExternalTaskMarker.__name__))
            for ti in external_tasks:
                ti_key = ti.key.primary
                if ti_key in visited_external_tis:
                    continue
                visited_external_tis.add(ti_key)
                task: ExternalTaskMarker = cast(ExternalTaskMarker, copy.copy(self.get_task(ti.task_id)))
                ti.task = task
                if max_recursion_depth is None:
                    
                    
                    max_recursion_depth = task.recursion_depth
                if recursion_depth + 1 > max_recursion_depth:
                    
                    raise AirflowException(
                        f"Maximum recursion depth {max_recursion_depth} reached for "
                        f"{ExternalTaskMarker.__name__} {ti.task_id}. "
                        f"Attempted to clear too many tasks or there may be a cyclic dependency."
                    )
                ti.render_templates()
                external_tis = session.scalars(
                    select(TI)
                    .join(TI.dag_run)
                    .where(
                        TI.dag_id == task.external_dag_id,
                        TI.task_id == task.external_task_id,
                        DagRun.execution_date == pendulum.parse(task.execution_date),
                    )
                )
                for tii in external_tis: