Support empty response for Completions and ChatCompletions API

[tboerstad]

This PR fixes a bug where evaluation will fail if any of the text/content responses is None from the API under test.
This happens because the parse_generations functions say that they return List[str], but will actually return a list[Optional[str]] when the response from the API is None.

This error scenario is more likely with reasoning models.
Some frameworks put the reasoning tokens in a different field, and without a high enough max_gen_toks the entire token budget will be spent on reasoning tokens. This leaves the text/content field as None.

I've hit this error scenario when testing gpt-oss-20b on vLLM with the gsm8k_cot_llama dataset.

[CLAassistant]

All committers have signed the CLA.

[baberabb]

Thanks for the PR! Left a comment to add warning in case an empty response is unexpected. Also if you could run the pre-commit for the formatting:

pip install pre-commit
pre-commit run --all-files
``

[tboerstad]

Thanks for the feedback. I've added a warning, tested that it's being emitted, and also ran the pre-commit run --all-files .