generate-examples fails if spec has circular reference

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.

Start by running `oav generate-examples` against the circular-reference example at the linked Azure REST API specs commit and inspect where the null warning becomes a failure. The issue names no source files, tests, or acceptance criteria, so the desired behavior for circular references must be clarified before implementation.

Written by the indexing model from the issue text.