### 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.
from airflow.models.taskinstance import (
    Context,
    TaskInstance,
    TaskInstanceKey,
    clear_task_instances,
)
from airflow.secrets.local_filesystem import LocalFilesystemBackend
from airflow.security import permissions
from airflow.stats import Stats
from airflow.timetables.base import DagRunInfo, DataInterval, TimeRestriction, Timetable
from airflow.timetables.interval import CronDataIntervalTimetable, DeltaDataIntervalTimetable
from airflow.timetables.simple import (
    ContinuousTimetable,
    DatasetTriggeredTimetable,
    NullTimetable,
    OnceTimetable,
)
from airflow.utils import timezone
from airflow.utils.dag_cycle_tester import check_cycle
from airflow.utils.dates import cron_presets, date_range as utils_date_range
from airflow.utils.decorators import fixup_decorator_warning_stack
from airflow.utils.helpers import at_most_one, exactly_one, validate_key
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import NEW_SESSION, provide_session
from airflow.utils.sqlalchemy import (
    Interval,
    UtcDateTime,
    lock_rows,
    skip_locked,
    tuple_in_condition,