### 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,
        dags,
        start_date=None,
        end_date=None,
        only_failed=False,
        only_running=False,
        confirm_prompt=False,
        include_subdags=True,
        include_parentdag=False,
        dag_run_state=DagRunState.QUEUED,
        dry_run=False,
    ):
        all_tis = []
        for dag in dags:
            tis = dag.clear(
                start_date=start_date,
                end_date=end_date,
                only_failed=only_failed,
                only_running=only_running,
                confirm_prompt=False,
                include_subdags=include_subdags,
                include_parentdag=include_parentdag,
                dag_run_state=dag_run_state,
                dry_run=True,