fix: S3 list_paths regression

[saikonen]

fixes a regression introduced in 2.19.6 that affects listing paths in the bucket root with the S3 client list_paths

[saikonen]

repro:

# test.py
import sys
from metaflow import S3


if __name__=="__main__":
    try:
      bucket = sys.argv[1]
      print("listing bucket:", bucket)
    except IndexError:
        print("Please provide a bucket name.")
        sys.exit(1)

    with S3(s3root=f"s3://{bucket}") as s3:
      objs = s3.list_paths()
      print([o.key for o in objs])

# lists nothing
python test.py your-metaflow-bucket

# lists just fine for subpaths
python test.py your-metaflow-bucket/metaflow

[nflx-mf-bot]

Netflix internal testing[1588] @ 9a74e50

[npow]

All internal S3 tests passing