fix: guard against empty/filtered LLM responses in make_me_say utils

[qizwiz]

What

Adds a null check in evals/elsuite/make_me_say/utils.py:get_content() before accessing response.choices[0].message.

Why

Two silent failure modes crash at this line:

1. IndexError — choices is an empty list ([]) when the provider returns no completions
2. AttributeError — choices[0].message is None when content is filtered (some providers return HTTP 200 with PROHIBITED_CONTENT finish reason)

Fix

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

Detected by pact static analysis.