### 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.
task = GCSObjectUpdateSensor(
            task_id="task-id",
            bucket=TEST_BUCKET,
            object=TEST_OBJECT,
            google_cloud_conn_id=TEST_GCP_CONN_ID,
            impersonation_chain=TEST_IMPERSONATION_CHAIN,
            deferrable=True,
        )
        mock_hook.return_value.is_updated_after.return_value = True
        task.execute(mock.MagicMock())
        assert not mock_defer.called
class TestGCSObjectUpdateAsyncSensor:
    OPERATOR = GCSObjectUpdateSensor(
        task_id="gcs-obj-update",
        bucket=TEST_BUCKET,
        object=TEST_OBJECT,
        google_cloud_conn_id=TEST_GCP_CONN_ID,
        deferrable=True,
    )
    @mock.patch("airflow.providers.google.cloud.sensors.gcs.GCSHook")
    def test_gcs_object_update_async_sensor(self, mock_hook):