### 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: DAG,
        last_automated_dag_run: None | datetime | DataInterval,
    ) -> None:
        """
        Calculate ``next_dagrun`` and `next_dagrun_create_after``.
        :param dag: The DAG object
        :param last_automated_dag_run: DataInterval (or datetime) of most recent run of this dag, or none
            if not yet scheduled.
        """
        last_automated_data_interval: DataInterval | None
        if isinstance(last_automated_dag_run, datetime):
            warnings.warn(
                "Passing a datetime to `DagModel.calculate_dagrun_date_fields` is deprecated. "
                "Provide a data interval instead.",
                RemovedInAirflow3Warning,
                stacklevel=2,
            )
            last_automated_data_interval = dag.infer_automated_data_interval(last_automated_dag_run)
        else:
            last_automated_data_interval = last_automated_dag_run