### 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.
DEFAULT_ARGS = {
        "owner": "test",
        "depends_on_past": True,
        "start_date": timezone.utcnow(),
        "retries": 1,
        "retry_delay": timedelta(minutes=1),
    }
    DEFAULT_DATE = timezone.datetime(2016, 1, 1)
    VALUE = 42
    def setup_method(self):
        self.operator = None
    def teardown_method(self):
        clear_db_runs()
    def test_fileloc(self):
        @dag_decorator(default_args=self.DEFAULT_ARGS)
        def noop_pipeline():