### 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.
:param dag_id: The id of the DAG; must consist exclusively of alphanumeric
        characters, dashes, dots and underscores (all ASCII)
    :param description: The description for the DAG to e.g. be shown on the webserver
    :param schedule: Defines the rules according to which DAG runs are scheduled. Can
        accept cron string, timedelta object, Timetable, or list of Dataset objects.
        If this is not provided, the DAG will be set to the default
        schedule ``timedelta(days=1)``. See also :doc:`/howto/timetable`.
    :param start_date: The timestamp from which the scheduler will
        attempt to backfill
    :param end_date: A date beyond which your DAG won't run, leave to None
        for open-ended scheduling
    :param template_searchpath: This list of folders (non-relative)
        defines where jinja will look for your templates. Order matters.
        Note that jinja/airflow includes the path of your DAG file by
        default
    :param template_undefined: Template undefined type.
    :param user_defined_macros: a dictionary of macros that will be exposed
        in your jinja templates. For example, passing ``dict(foo='bar')``
        to this argument allows you to ``{{ foo }}`` in all jinja
        templates related to this DAG. Note that you can pass any
        type of object here.
    :param user_defined_filters: a dictionary of filters that will be exposed
        in your jinja templates. For example, passing
        ``dict(hello=lambda name: 'Hello %s' % name)`` to this argument allows
        you to ``{{ 'world' | hello }}`` in all jinja templates related to
        this DAG.
    :param default_args: A dictionary of default parameters to be used
        as constructor keyword parameters when initialising operators.
        Note that operators have the same hook, and precede those defined
        here, meaning that if your dict contains `'depends_on_past': True`