Idle executor should properly be killed when no job is submitted

I enable dynamic allocation feature with spark-shell and do not submit any task. After spark.dynamicAllocation.executorIdleTimeout seconds(default 60s), there is still one active executor, which is abnormal. All idle executors are timeout and should be removed.(default spark.dynamicAllocation.minExecutors=0). The spark-shell command show below:

spark-shell --master=yarn --conf spark.ui.port=8040 --conf spark.dynamicAllocation.enabled=true --conf spark.dynamicAllocation.maxExecutors=8 --conf spark.dynamicAllocation.initialExecutors=4 --conf spark.shuffle.service.enabled=true