Fix  ExternalServerError for None URLs in split-opt-in-out-scan
Fix for https://github.com/huggingface/datasets-server/issues/2608
Many URLs are None and the job runner failed when doing the join operation at https://github.com/huggingface/datasets-server/blob/main/services/worker/src/worker/job_runners/split/opt_in_out_urls_scan_from_streaming.py#L38
Fix  ExternalServerError for None URLs in split-opt-in-out-scan: Fix for https://github.com/huggingface/datasets-server/issues/2608
Many URLs are None and the job runner failed when doing the join operation at https://github.com/huggingface/datasets-server/blob/main/services/worker/src/worker/job_runners/split/opt_in_out_urls_scan_from_streaming.py#L38