Fix kube-apiserver crash when patching TPR data

[nikhita]

What this PR does / why we need it: While trying to do a strategic merge patch, kube-apiserver crashes with a nil pointer dereference because UnsafeConvertor is nil for TPR data resources here: https://github.com/kubernetes/kubernetes/blob/master/pkg/master/thirdparty/thirdparty.go#L314-L337.

This PR does not make strategic merge patch work on TPR data but makes sure that the kube-api server does not crash while trying to do so.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #44587

Special notes for your reviewer:

Release note:

Fix kube-apiserver crash when attempting an unsupported strategic merge patch operation on TPR data.

[k8s-reviewable]

This change is 

[k8s-ci-robot]

Hi @nikhita. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with @k8s-bot ok to test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[xiang90]

@nikhita

can you add a simple test to ensure this wont happen in the future. thanks.

[nikhiljindal]

@k8s-bot ok to test

cc @kubernetes/sig-api-machinery-pr-reviews

[nikhita]

@k8s-bot non-cri e2e test this

[deads2k]

@erictune are you and @foxish still planning on legacy TPR work? This is probably worth chasing up.

[foxish]

@deads2k, @enisoc will be taking over the TPR work I believe.

[enisoc]

@nikhita Do you have time to add a test for this? If so, I think it could go here: https://github.com/kubernetes/kubernetes/blob/master/test/integration/thirdparty/thirdparty_test.go

[sttts]

Any reason not to merge this? Even without a test, this has value and fixes a crash.

[sttts]

/lgtm

[sttts]

/approve

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nikhita, sttts

Needs approval from an approver in each of these OWNERS Files:

• pkg/master/OWNERS [sttts]

You can indicate your approval by writing /approve in a comment
You can cancel your approval by writing /approve cancel in a comment

[k8s-github-robot]

Automatic merge from submit-queue

[k8s-github-robot]

@nikhita PR needs rebase

[philips]

Can this be cherry-picked to v1.6.5?

[philips]

@ericchiang put up a cherry-pick #46389