Add a check on GeoCentroidAgg for the NEST client 5.x when analyzing StatsAgg

[sergey-borisenko]

NEST/Elasticsearch.Net version: < 6.x

Elasticsearch version: >= 5.5.0

Description of the problem including expected versus actual behavior:
The NullReferenceException occurre when the GeoCentroid aggregation does not have any documents.

Steps to reproduce:

1. You should have:
   a) Elasticsearch with version greater or equal 5.5.0
   b) NEST client (for example 5.6.3)

2. Create Index with this mapping

{
  "coordinate": {
    "type": "geo_point"
  },
  "color": {
    "type": "keyword"
  }
}

3. Then send request (any request wich return empty search context). We have empty index, so in our case, any request.

{
  "size": 0,
  "query": {
    "term": {
      "color": "black"
    }
  },
  "aggs": {
    "geo_centroid_agg": {
      "geo_centroid": {
        "field": "coordinate"
      }
    }
  }
}

Result for version >= 5.5.0:

{
  "took": 2,
  "timed_out": false,
  "_shards": {
    "total": 3,
    "successful": 3,
    "failed": 0
  },
  "hits": {
    "total": 0,
    "max_score": 0,
    "hits": []
  },
  "aggregations": {
    "geo_centroid_agg": {
      "count": 0
    }
  }
}

Result for version < 5.5.0:

{
  "took": 2,
  "timed_out": false,
  "_shards": {
    "total": 3,
    "successful": 3,
    "failed": 0
  },
  "hits": {
    "total": 0,
    "max_score": 0,
    "hits": []
  },
  "aggregations": {
    "geo_centroid_agg": {}
  }
}

4. You got the NullReferenceException

at Nest.AggregateJsonConverter.GetStatsAggregate(Newtonsoft.Json.JsonReader reader, Newtonsoft.Json.JsonSerializer serializer) at offset 252
at Nest.AggregateJsonConverter.ReadAggregate(Newtonsoft.Json.JsonReader reader, Newtonsoft.Json.JsonSerializer serializer) at offset 504
at Nest.AggregateJsonConverter.GetSubAggregates(Newtonsoft.Json.JsonReader reader, Newtonsoft.Json.JsonSerializer serializer) at offset 43
at Nest.AggregateJsonConverter.GetSingleBucketAggregate(Newtonsoft.Json.JsonReader reader, Newtonsoft.Json.JsonSerializer serializer) at offset 264
at Nest.AggregateJsonConverter.ReadAggregate(Newtonsoft.Json.JsonReader reader, Newtonsoft.Json.JsonSerializer serializer) at offset 515
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(Newtonsoft.Json.JsonConverter converter, Newtonsoft.Json.JsonReader reader, System.Type objectType, System.Object existingValue) at offset 74

Without property "location" the property "count" interpreted as the begin of StatsAggregate.
I think that the history of this problem began in this task elastic/elasticsearch#24387

Please, add this fix https://github.com/elastic/elasticsearch-net/blame/52566943f3e0bf70f747ff8e35f70c303c56e1e3/src/Nest/Aggregations/AggregateJsonConverter.cs#L338
for next minor version 5.x

[russcam]

relates: #3000

[russcam]

Closing this as #3347 has been merged for the next 5.x release