### 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.
"idx_dag_run_queued_dags",
            "state",
            "dag_id",
            postgresql_where=text("state='queued'"),
            mssql_where=text("state='queued'"),
            sqlite_where=text("state='queued'"),
        ),
    )
    task_instances = relationship(
        TI, back_populates="dag_run", cascade="save-update, merge, delete, delete-orphan"
    )
    dag_model = relationship(
        "DagModel",
        primaryjoin="foreign(DagRun.dag_id) == DagModel.dag_id",
        uselist=False,
        viewonly=True,
    )
    dag_run_note = relationship(
        "DagRunNote",
        back_populates="dag_run",
        uselist=False,
        cascade="all, delete, delete-orphan",
    )
    note = association_proxy("dag_run_note", "content", creator=_creator_note)
    DEFAULT_DAGRUNS_TO_EXAMINE = airflow_conf.getint(
        "scheduler",
        "max_dagruns_per_loop_to_schedule",
        fallback=20,
    )
    def __init__(