Problem

OpenAI-compatible APIs can fail silently in two ways:

1. Empty choices list (IndexError): response.choices is [] when the API returns no completions
2. message=None (AttributeError): response.choices[0].message is None when content is filtered (e.g., Gemini returns HTTP 200 with PROHIBITED_CONTENT, message=None)

Both paths cause unhandled runtime exceptions during translation, aborting the entire translation job.

Affected code

Three Translator subclasses in pdf2zh/translator.py have unguarded accesses (4 sites total — lines 549, 628, 631, 1198 in the original).

Fix

Adds the standard two-condition guard before each bare choices[0].message.content access:

if not response.choices or response.choices[0].message is None:
    raise ValueError("LLM returned empty or filtered response")

This converts silent crashes into explicit, actionable errors with a clear message.

8 lines added, 0 deleted.

Verification

Detected by pact static analysis (llm_response_unguarded rule). Confirmed: Gemini 2.5 Flash returns HTTP 200 with choices[0].message=None on prohibited content.