Fix elasticsearch output adding bulk index item

Error:

```
{"log.level":"debug","@timestamp":"2023-02-21T21:16:52.852Z","log.logger":"elasticsearch-output","log.origin":{"file.name":"elasticsearch/output.go","file.line":137},"message":"Failed to add items: \"\"","service.name":"shipper","ecs.version":"1.6.0"}
```

most likely due to b5bdbfefe13cd9c58023fd04df36bda8cb9c7e88, was working at 4d8899c701c43f4495044498a5a4e5dec17d4016
