@Override
  public Optional<ExtendedCell> getLastKey() {
    return dataBlockIndexReader.isEmpty()
      ? Optional.empty()
      : Optional.of(fileInfo.getLastKeyCell());
  }
@return Last key as cell in the file. May be null if file has no entries. Note that this is not
        the last row key, but it is the Cell representation of the last key
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java