Adapt has_labels test when no labels were found
# What does this PR do?

As #20105 highlights it, the new way we infer default label names for models might not work for models outside of Transformers This PR reverts to the old default for non-`PreTrainedModel` models
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20113). All of your documentation changes will be reflected on that endpoint.