### 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 = GCSObjectsWithPrefixExistenceSensor(
            task_id="task-id",
            bucket=TEST_BUCKET,
            prefix=TEST_PREFIX,
            google_cloud_conn_id=TEST_GCP_CONN_ID,
            impersonation_chain=TEST_IMPERSONATION_CHAIN,
            poke_interval=0,
        )
        generated_messages = [f"test-prefix/obj{i}" for i in range(5)]
        mock_hook.return_value.list.return_value = generated_messages
        response = task.execute(None)
        mock_hook.assert_called_once_with(
            gcp_conn_id=TEST_GCP_CONN_ID,
            impersonation_chain=TEST_IMPERSONATION_CHAIN,
        )
        mock_hook.return_value.list.assert_called_once_with(TEST_BUCKET, prefix=TEST_PREFIX)
        assert response == generated_messages
    @pytest.mark.parametrize(
        "soft_fail, expected_exception", ((False, AirflowSensorTimeout), (True, AirflowSkipException))
    )
    @mock.patch("airflow.providers.google.cloud.sensors.gcs.GCSHook")
    def test_execute_timeout(self, mock_hook, soft_fail, expected_exception):