Cleanup does not remove empty directories

If I create an index and then delete it I am left with an empty directory (named after the index uuid)

import lance
import shutil
import pyarrow as pa
import glob
import datetime

shutil.rmtree("/tmp/test.lance", ignore_errors=True)

ds = lance.write_dataset(pa.table({"x": range(1000)}), "/tmp/test.lance")

num_folders = len(glob.glob("/tmp/test.lance/_indices/*"))
num_files = len(glob.glob("/tmp/test.lance/_indices/**/*.lance"))
print(f"Before creating an index there are {num_folders} folders and {num_files} files in the _indices directory")

ds.create_scalar_index("x", "BTREE")

num_folders = len(glob.glob("/tmp/test.lance/_indices/*"))
num_files = len(glob.glob("/tmp/test.lance/_indices/**/*.lance"))
print(f"After creating an index there are {num_folders} folders and {num_files} files in the _indices directory")

ds.drop_index("x_idx")
num_folders = len(glob.glob("/tmp/test.lance/_indices/*"))
num_files = len(glob.glob("/tmp/test.lance/_indices/**/*.lance"))
print(f"After dropping an index there are {num_folders} folders and {num_files} files in the _indices directory")

ds.cleanup_old_versions(older_than=datetime.timedelta(days=0))
num_folders = len(glob.glob("/tmp/test.lance/_indices/*"))
num_files = len(glob.glob("/tmp/test.lance/_indices/**/*.lance"))
print(f"After cleanup there are {num_folders} folders and {num_files} files in the _indices directory")

The output is:

Before creating an index there are 0 folders and 0 files in the _indices directory
After creating an index there are 1 folders and 2 files in the _indices directory
After dropping an index there are 1 folders and 2 files in the _indices directory
After cleanup there are 1 folders and 0 files in the _indices directory

This only matters when using local disk and not object storage.

Reproduce the local-disk case using the Python sequence in the issue, focusing on create_scalar_index, drop_index, and cleanup_old_versions under the _indices directory. Trace how cleanup removes index files and verify the local directory is also removed when it becomes empty; the existing reproduction should end with no leftover index directory.