Automatic merge from submit-queue (batch tested with PRs 54837, 55970, 55912, 55898, 52977). 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 protobuf generation for empty struct

Currently, we are not able to generate protobuf for empty structs. This PR fixes proto generation for empty structs. (Example: `type Foo struct{}`)

Needed for https://github.com/kubernetes/kubernetes/pull/55168.

**Release note**:

```release-note
NONE
```

/assign @sttts