runaway query watch=plan may record empty plan, which cause failed connection

[nolouch]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

ALTER RESOURCE GROUP rg_test RU_PER_SEC = 1000 QUERY_LIMIT=(EXEC_ELAPSED '10ms' ACTION KILL WATCH =PLAN)
use rg_test;

2. What did you expect to see? (Required)

can connect to db

3. What did you see instead (Required)

cannot connect to db.

4. What is your TiDB version? (Required)

master