fix(ragas): score 0 when Faithfulness gets no statements

[fei (feiiiiii5)]

When the LLM judge returns no statements — or no verdicts for the statements it did return — Faithfulness divided by len(faithfulness) with an empty list, so a single unfaithful-looking answer killed the whole eval run instead of scoring it:

Faithfulness().eval(input="What is the capital of France?", output="", context="Paris is the capital of France.")
# ZeroDivisionError: division by zero  (py/autoevals/ragas.py:998)

The TypeScript scorer already handles this case, in js/ragas.ts:

const score = faithfulness.length
  ? faithfulness.reduce((acc, { verdict }) => acc + verdict, 0) / faithfulness.length
  : 0;

This applies the same guard to the Python async and sync paths, so an empty verdict list scores 0 and the run continues.

Test: pytest py/autoevals/test_ragas.py -k no_statements — the new test fails on 9546b28 with ZeroDivisionError: division by zero at py/autoevals/ragas.py:998 (both the sync and async parametrization) and passes on this branch. The rest of test_ragas.py is unchanged: 9 of its tests fail identically before and after this branch because they call a real OpenAI model and this environment has no API key.

Also ran the repo's configured lint on the two changed files: black --check (clean), ruff check (clean), codespell (clean).

Details

AnswerRelevancy has the same unguarded division at py/autoevals/ragas.py:1152 (/ len(questions)), but the TypeScript twin is unguarded in the same place, so there is no prior art to point at. I left it alone rather than fold a second, differently-evidenced change into this one.

py/autoevals/ragas.py:402 (ContextRelevancy, / len(context)) is likewise unguarded in both languages; that one divides by user input rather than a judge result, so it is a different shape again.