### 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.
OPERATOR = GCSUploadSessionCompleteSensor(
        task_id="gcs-obj-session",
        bucket=TEST_BUCKET,
        google_cloud_conn_id=TEST_GCP_CONN_ID,
        prefix=TEST_OBJECT,
        inactivity_period=TEST_INACTIVITY_PERIOD,
        min_objects=TEST_MIN_OBJECTS,
        deferrable=True,
    )
    @mock.patch("airflow.providers.google.cloud.sensors.gcs.GCSHook")
    def test_gcs_upload_session_complete_async_sensor(self, mock_hook):