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quote valid strings for field validation

**What this PR does / why we need it**:
Some of the declared const vars are empty string `""`, such as

* [`NamespaceAll`, `NamespaceNone`](https://github.com/kubernetes/kubernetes/blob/master/pkg/api/types.go#L197-L200)
* [`HostPathUnset`](https://github.com/kubernetes/kubernetes/blob/master/pkg/api/types.go#L631)

When validating such supported fields, empty string will be printed without quotes, which is kind of ugly.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #53849

**Special notes for your reviewer**:
/assign @liggitt 

**Release note**:

```release-note
None
```