@Override
  public Optional<byte[]> getFirstRowKey() {
    // We have to copy the row part to form the row key alone
    return getFirstKey().map(CellUtil::cloneRow);
  }
TODO left from {@link HFile} version 1: move this to StoreFile after Ryan's patch goes in to
eliminate {@link KeyValue} here.
@return the first row key, or null if the file is empty.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java