Reject malformed JSON instead of treating it as empty

[primorLee]

Summary

• distinguish a valid empty JSON object from extract_json's failure sentinel
• reject malformed model output that merely contains opening and closing braces
• cover valid empty objects and malformed object/list shapes with regression tests

Testing

• uv run --isolated --with pytest python -m pytest tests/unit/test_llm_chat.py -k coerce_to_dict_or_list -o addopts= -q (3 passed)
• python -m compileall -q tinytroupe/utils/llm.py

Fixes #159