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openapi: Validate unregistered type, if they can be found

**What this PR does / why we need it**:
Types that are not registered/hard-coded in kubectl won't be validated, even if they could because they are defined in openapi. If they are neither registered nor in openapi, then skip validation. 

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes nothing

**Special notes for your reviewer**:

**Release note**:
```release-note
NONE
```