[AIRFLOW-5798] Set default ExternalTaskSensor.external_task_id (#6431)
[AIRFLOW-5798] Set default ExternalTaskSensor.external_task_id (#6431)

- fix type annotation for ExternalTaskSensor

(cherry picked from commit 3fb9d4935f5d30feb4c077f17fdbda04bc195e90)

owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow