fix: grade None provider completions as empty answers instead of crashing

[lezama]

What

First real multi-model pilot run (gemini-2.5-pro + flash on the hardened dataset) crashed with:

File .../wp_bench/utils.py, line 9, in strip_code_fences
    stripped = output.strip()
AttributeError: 'NoneType' object has no attribute 'strip'

litellm can hand back a choice whose message.content is None — providers do this on safety blocks, thinking-only responses, or truncation. ModelGeneration.text carried the None into parse_artifact, killing the entire run on one bad completion.

Fix

Normalize at the single extraction point in ModelInterface.generate_with_metadata: content or "". An empty completion is a model output, not a harness fault, so it should grade as an empty answer (score 0 for that test) and let the run continue. This also composes with run.continue_on_error for the remaining per-test failure modes.

Includes a regression test with a mocked None-content response.

🤖 Generated with Claude Code

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Co-authored-by: lezama <migueluy@git.wordpress.org>

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