generate-examples fails if spec has circular reference

[mikeharder]

If a spec has a circular reference, oav generate-examples will fail with a warning foo is null, it may be caused by circular reference. An example is this PR as of this commit:

https://github.com/Azure/azure-rest-api-specs-pr/pull/19372/commits/b064b54deeaef1a7647afa036e2a223e80f38b7a

It's common for OpenAPI 2.0 tools to fail on specs with circular references, so this might not be worth fixing, but we can still use this issue for tracking.