### 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 **downstream** selected."""
        upstream = False
        return set(
            task.dag.partial_subset(
                task_ids_or_regex=[task.task_id],
                include_downstream=not upstream,
                include_upstream=upstream,
            ).tasks
        )
    @staticmethod
    def cleared_upstream(task):
        """Helper to return tasks that would be cleared if **upstream** selected."""
        upstream = True
        return set(
            task.dag.partial_subset(
                task_ids_or_regex=task.task_id,
                include_downstream=not upstream,
                include_upstream=upstream,
            ).tasks
        )
    @staticmethod
    def cleared_neither(task):