code: public Optional<List<FieldSchema>> getPartitionKeys() {
      String serialzed = this.getProp(HIVE_PARTITION_KEYS);
      if (serialzed == null) {
        return Optional.absent();
      }
      List<FieldSchema> deserialized = GSON.fromJson(serialzed, FIELD_SCHEMA_TYPE);
      return Optional.of(deserialized);
    }
docs: Get the partition keys if this {@link WorkUnit} is for a partitioned table.
  If not, return {@link Optional#absent()}