### 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.
here and `'depends_on_past': False` in the operator's call
        `default_args`, the actual value will be `False`.
    :param params: a dictionary of DAG level parameters that are made
        accessible in templates, namespaced under `params`. These
        params can be overridden at the task level.
    :param max_active_tasks: the number of task instances allowed to run
        concurrently
    :param max_active_runs: maximum number of active DAG runs, beyond this
        number of DAG runs in a running state, the scheduler won't create
        new active DAG runs
    :param dagrun_timeout: specify how long a DagRun should be up before
        timing out / failing, so that new DagRuns can be created.
    :param sla_miss_callback: specify a function or list of functions to call when reporting SLA
        timeouts. See :ref:`sla_miss_callback<concepts:sla_miss_callback>` for
        more information about the function signature and parameters that are
        passed to the callback.
    :param default_view: Specify DAG default view (grid, graph, duration,
                                                   gantt, landing_times), default grid
    :param orientation: Specify DAG orientation in graph view (LR, TB, RL, BT), default LR
    :param catchup: Perform scheduler catchup (or only run latest)? Defaults to True
    :param on_failure_callback: A function or list of functions to be called when a DagRun of this dag fails.
        A context dictionary is passed as a single parameter to this function.
    :param on_success_callback: Much like the ``on_failure_callback`` except
        that it is executed when the dag succeeds.
    :param access_control: Specify optional DAG-level actions, e.g.,
        "{'role1': {'can_read'}, 'role2': {'can_read', 'can_edit', 'can_delete'}}"
    :param is_paused_upon_creation: Specifies if the dag is paused when created for the first time.
        If the dag exists already, this flag will be ignored. If this optional parameter
        is not specified, the global config setting will be used.
    :param jinja_environment_kwargs: additional configuration options to be passed to Jinja