Merge pull request #57830 from hzxuzhonghu/buf-fix

Automatic merge from submit-queue (batch tested with PRs 56971, 57570, 57830, 57742). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

fix possible panic in kube-aggregator

**What this PR does / why we need it**:

https://github.com/kubernetes/kubernetes/blob/91dc55562c9e7742a1cfdaa6d1db0b1ce262ff95/staging/src/k8s.io/kube-aggregator/pkg/apiserver/handler_proxy.go#L210

If startup kube-aggregator as a standalone process. `r.proxyTransport ` is uninitialized, so `r.proxyTransport.Dial` will panic.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```

Kubernetes-commit: 4cadc31965195d86f6658ae878989fbe461bd2ca