yatp: reschedule only if there are awaiting tasks (#6981)
yatp: reschedule only if there are awaiting tasks (#6981)

Signed-off-by: Yilin Chen <9f6bed1fd69d0e15c5d9fbf33319f3d0f8cbc703@gmail.com>
c4pt0r/tikv,pingcap/tikv,BusyJay/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv