fix: guard against empty/filtered LLM responses in groqclient

[qizwiz]

What

Adds a null check in omnitool/gradio/agent/llm_utils/groqclient.py before accessing completion.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 a filtered finish reason)

Fix

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

Detected by pact static analysis.