### 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.
dag_id: str,
        execution_date: datetime | None = None,
        run_id: str | None = None,
        session: Session = NEW_SESSION,
    ) -> DagRun | DagRunPydantic:
        """
        Return the dag run for a given execution date or run_id if it exists, otherwise none.
        :param dag_id: The dag_id of the DAG to find.
        :param execution_date: The execution date of the DagRun to find.
        :param run_id: The run_id of the DagRun to find.
        :param session:
        :return: The DagRun if found, otherwise None.
        """
        if not (execution_date or run_id):
            raise TypeError("You must provide either the execution_date or the run_id")
        query = select(DagRun)
        if execution_date:
            query = query.where(DagRun.dag_id == dag_id, DagRun.execution_date == execution_date)
        if run_id:
            query = query.where(DagRun.dag_id == dag_id, DagRun.run_id == run_id)
        return session.scalar(query)
    @provide_session
    def get_dagrun(