private Pair<byte[], byte[]> mapEntryToPair(final Map.Entry<Bytes, Optional<byte[]>> entry) {
      byte[] value = entry.getValue().orElse(null);
      return Pair.of(entry.getKey().toArrayUnsafe(), value);
    }
Constructor which takes an explicit backing map for the layered key value storage.

@param map the backing map
@param parent the parent key value storage for this layered storage.
services/kvstore/src/main/java/org/hyperledger/besu/services/kvstore/LayeredKeyValueStorage.java