### 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.
self,
        dag_id: str | None = None,
        run_id: str | None = None,
        queued_at: datetime | None | ArgNotSet = NOTSET,
        execution_date: datetime | None = None,
        start_date: datetime | None = None,
        external_trigger: bool | None = None,
        conf: Any | None = None,
        state: DagRunState | None = None,
        run_type: str | None = None,