fix: handle empty completions in parse_answer

[JannikSt]

Fixes crash when API times out and returns empty completion.

• Return None instead of crashing with IndexError when no assistant messages
• Added test cases for empty completions

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Note

Prevents crashes when completions lack assistant messages.

• Update parse_answer to safely handle empty lists or completions without assistant messages by returning None
• Add tests: test_parse_answer_with_empty_completion and test_parse_answer_with_no_assistant_messages to validate behavior

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