Add test case for nullable parent property

[martin-mfg]

This is a follow-up for #16497

I added a testcase for the problem fixed in above PR to .../resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml and .../resources/3_0/spring/petstore-with-fake-endpoints-models-for-testing.yaml.

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[martin-mfg]

The failing JAX-RS test seems to be unrelated, because my PR doesn't change anything in /samples/server/petstore/jaxrs-cxf.

[wing328]

agreed. I could reproduce the issue in the master. will try to fix it in another PR.