record zero-length files in the shard serialization manifest

[Support-itecz]

Summary

_get_shards guards its loop with if path_size > 0, so a zero-length file produces no shards and never becomes a manifest item. Shard serialization leaves every empty file in the model out of the signature, even though the file was there when the model was signed.

The verifier re-serializes the model the same way, so the omission is symmetric and nothing reports a difference. Against a model signed with use_shard_serialization():

• deleting a signed empty file verifies clean
• adding new empty files verifies clean, including one that creates a new directory (sub/__init__.py)
• with ignore_unsigned_files, a signed-as-empty file can be filled with content and still verifies clean

Empty marker files decide real loader behavior (__init__.py, py.typed, feature-flag files), so this is a gap in what the signature covers rather than a cosmetic difference. File serialization already recorded empty files, which is why file_test.py asserts test_folder_model_empty_file_gets_included while the shard suite asserted the opposite and the shard goldens for both empty-file fixtures were zero-byte. Dropping the guard makes _endpoints(shard_size, 0) yield one (path, 0, 0) shard, and ShardedFileHasher.set_shard accepts end == start for it, so the two serializers agree.

Shard signatures over models that contain empty files have to be regenerated. The unit suite passes (198), and the version-compat verify scripts under scripts/tests still pass for v0.2.0 through v1.1.0, since those signatures are file-serialized.

Checklist

• All commits are signed-off, using DCO
• All new code has docstrings and type annotations
• All new code is covered by tests. Aim for at least 90% coverage. CI is configured to highlight lines not covered by tests.
• Public facing changes are paired with documentation changes
• Release note has been added to CHANGELOG.md if needed