@Override
    public void add(byte[] firstKey, long blockOffset, int onDiskDataSize) {
      add(firstKey, blockOffset, onDiskDataSize, -1);
    }
The same as {@link #add(byte[], long, int, long)} but does not take the key/value into
account. Used for single-level indexes.
@see #add(byte[], long, int, long)
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlockIndex.java