### 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.
"""Create a Timetable instance from a ``schedule_interval`` argument."""
    if interval is NOTSET:
        return DeltaDataIntervalTimetable(DEFAULT_SCHEDULE_INTERVAL)
    if interval is None:
        return NullTimetable()
    if interval == "@once":
        return OnceTimetable()
    if interval == "@continuous":
        return ContinuousTimetable()
    if isinstance(interval, (timedelta, relativedelta)):
        return DeltaDataIntervalTimetable(interval)
    if isinstance(interval, str):
        return CronDataIntervalTimetable(interval, timezone)
    raise ValueError(f"{interval!r} is not a valid interval.")
def get_last_dagrun(dag_id, session, include_externally_triggered=False):
    """
    Return the last dag run for a dag, None if there was none.
    Last dag run can be any type of run e.g. scheduled or backfilled.
    Overridden DagRuns are ignored.
    """
    DR = DagRun
    query = select(DR).where(DR.dag_id == dag_id)
    if not include_externally_triggered:
        query = query.where(DR.external_trigger == expression.false())
    query = query.order_by(DR.execution_date.desc())
    return session.scalar(query.limit(1))
def get_dataset_triggered_next_run_info(