Some OpenAI-compatible providers return null for top_logprobs when
logprobs=true but top_logprobs is unset or 0. The OpenAI spec requires
top_logprobs to be Array<TopLogprob> (never null), so this triggers
Pydantic validation errors while parsing responses.
Add a Pydantic v2 field_validator on ChatCompletionTokenLogprob that
normalizes None -> [] before type validation. This preserves the typed
List[TopLogprob] contract for downstream consumers while remaining
narrowly scoped to null only (other invalid types are still rejected).
Fixes#21932