fix: handle empty YAML and JSON config files

[DarshanHarihar1]

Summary

Loading an empty justllms.yaml / justllms.yml or a JSON file containing only null caused Config.from_file() to crash with TypeError: argument after ** must be a mapping, not NoneType, because yaml.safe_load() and json.load() return None for those inputs.

This change treats null/empty documents as an empty configuration mapping (same as {}) and raises a clear ValueError when the top-level document is not a mapping (for example, a YAML list).

Changes

• Config.from_file() now normalizes None to {} before constructing Config
• Non-mapping top-level values raise ValueError with a descriptive message

Testing

Ran the full CI workflow locally:

• ruff check justllms/
• black --check justllms/
• mypy justllms/ --ignore-missing-imports
• python -m build + wheel import smoke test

Verified manually:

• Empty .yaml → returns Config() with default empty providers
• .json containing null → same behavior
• YAML list at top level → ValueError