code: private static String sanitizeDatasetUrn(String datasetUrn) {
      return Strings.isNullOrEmpty(datasetUrn) ? DEFAULT_DATASET_URN : HadoopUtils.sanitizePath(datasetUrn, "_");
    }
docs: Replace a null or empty dataset URN with {@link #DEFAULT_DATASET_URN}, and replaces illegal HDFS
  characters with '_'.