`TestResourceManagerClientTestSuite` panic

## Flaky Test

### Which jobs are failing
```
[2024/11/08 03:55:04.067 +00:00] [FATAL] [log.go:92] [panic] [recover="\"invalid memory address or nil pointer dereference\""] [stack="github.com/tikv/pd/pkg/utils/logutil.LogPanic\n\t/home/runner/work/pd/pd/pkg/utils/logutil/log.go:92\nruntime.gopanic\n\t/opt/hostedtoolcache/go/1.23.2/x64/src/runtime/panic.go:785\nruntime.panicmem\n\t/opt/hostedtoolcache/go/1.23.2/x64/src/runtime/panic.go:262\nruntime.sigpanic\n\t/opt/hostedtoolcache/go/1.23.2/x64/src/runtime/signal_unix.go:900\ngithub.com/tikv/pd/pkg/mcs/resourcemanager/server.(*ResourceGroup).getFillRate\n\t/home/runner/work/pd/pd/pkg/mcs/resourcemanager/server/resource_group.go:120\ngithub.com/tikv/pd/pkg/mcs/resourcemanager/server.(*Manager).backgroundMetricsFlush\n\t/home/runner/work/pd/pd/pkg/mcs/resourcemanager/server/manager.go:480"]
```

### CI link
https://github.com/tikv/pd/actions/runs/11722005088/job/32693952735

### Reason for failure (if possible)

### Anything else