fix: report an error when the fetched openapi schema is not JSON

[PragalvaXFREZ]

printSchema runs kubectl get --raw /openapi/v2 and unmarshals stdout. The json.Unmarshal error is discarded, not even assigned to _:

var jsonSchema map[string]interface{}
output := stdout.Bytes()
json.Unmarshal(output, &jsonSchema)
output, _ = json.MarshalIndent(jsonSchema, "", "  ")

When kubectl succeeds but the response is not JSON, for example an auth portal or corporate proxy returning HTML, jsonSchema is left nil, MarshalIndent renders it as null, and the command prints that and exits 0:

$ kustomize openapi fetch
null
$ echo $?
0

The function already guards the two neighbouring failures, kubectl exiting non-zero and empty stdout, and both reuse the errMsg advice. Only "kubectl succeeded but returned something that is not a schema" was unguarded, so a scripted fetch writes null to a file and nothing notices.

After the fix:

$ kustomize openapi fetch
Error: unable to parse the schema returned by kubectl: invalid character '<' looking for beginning of value
Error fetching schema from cluster.
Please make sure kubectl is installed, its context is set correctly, and your cluster is up.
Installation and setup instructions: https://kubernetes.io/docs/tasks/tools/install-kubectl/
$ echo $?
1

A valid schema still round-trips unchanged in both --format=json and --format=yaml. Reproduced with a stub kubectl on PATH.

[kubernetes-prow]

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This pull-request has been approved by: PragalvaXFREZ
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[kubernetes-prow]

Hi @PragalvaXFREZ. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /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.

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[koba1t]

/ok-to-test