KuzzleDataMapping.mapping is undefined when retrieving an empty mapping

[scottinet]

It's possible to have empty mappings in elasticsearch:

{
  "index": {
    "mappings": {
      "collection": {}
    }
  }
}

The SDK expects a properties attributes in the object returned by Elasticsearch, but in this case, there is none. And the SDK ends up building a KuzzleDataMapping object with an undefined mapping, causing unpredictable behaviors.