The config is not persisted after the balance-hot-region-scheduler is started

[HunDunDM]

Bug Report

What did you do?

• Start a cluster
• ETCDCTL_API=3 tiup ctl:{} etcd get --endpoints={} --prefix "/pd/{}/scheduler_config/balance-hot-region-scheduler"

What did you expect to see?

• Json config string

What did you see instead?

• null

What version of PD are you using (pd-server -V)?

v6.3.0