fix(iota-indexer): fix deserialization warning in `HttpRestKVClient`

[sergiupopescu199]

The cause of this issue is because when the value is not found on the KV REST API we return an empty body, but the response is a 204 which is a successful one, then the fetch method maps it as Ok(Some(bytes)), doing a deserialization of bytes of len=0 triggers that warming message.

A potential fix will be to modify the async fn fetch(&self, key: Key) -> IndexerResult<Option<Bytes>> to return Ok(None) if the body is empty.