The model can send `"path": null` as easily as `"path": ""`; on base a
None path reached search_tool and surfaced as an os.PathLike TypeError
instead of the documented current-directory default. Widen the blank-path
normalisation in _handle_search_files to any non-string and pin it in the
existing parametrized test.
Co-authored-by: joaomarcos <joaomarcosdias444@gmail.com>