Handle empty CSV exports in remote dataset cache

[biefan]

Summary

• allow empty example lists to be written through the CSV cache/export helper
• add a regression test covering _RemoteDatasetLoader._write_cache(..., file_type="csv") with no rows

Problem

_RemoteDatasetLoader routes CSV cache writes through pyrit.common.csv_helper.write_csv(). That helper currently assumes examples[0] exists, so exporting or caching an empty CSV-backed dataset raises IndexError instead of producing an empty file like the JSON, JSONL, and TXT helpers already do.

Testing

• .venv/bin/pytest tests/unit/datasets/test_remote_dataset_loader.py -q