Allow listing an empty drive root

[gaoflow]

Summary

• treat an empty top-level Drive listing as an empty directory instead of a missing path
• keep FileNotFoundError for missing non-root paths
• add unit tests for both paths

Tests

• .venv/bin/python -m pytest -q
• .venv/bin/pre-commit run --files gdrive_fsspec/core.py tests/test_core.py

Fixes #59

AI assistance was used under my direction.

[martindurant]

What happens if the drive in question doesn't exist at all? I would expect FileNotFound rather than an empty listing.

[gaoflow]

Good point. I updated the branch so the empty-root case and missing-root case are distinct.

Current behavior after 01ede5e:

• ls("") on an existing but empty default/root folder still returns [].
• ls("") with a custom root_file_id that exists but has no children still returns [].
• ls("") with a custom root_file_id whose listing is empty and whose id cannot be resolved now raises FileNotFoundError instead of caching/returning an empty listing.

I also rebased the branch onto the latest master, which should resolve the dirty merge state.

Verification:

.venv/bin/python -m pytest tests/test_core.py -q
# 31 passed, 2 deselected in 0.06s

.venv/bin/python -m pytest -q
# 31 passed, 2 deselected in 0.06s

ruff check gdrive_fsspec/core.py tests/test_core.py
# All checks passed!

ruff format --check gdrive_fsspec/core.py tests/test_core.py
# 2 files already formatted

git diff --check origin/master...HEAD
# no output