Fix Stats aggregate sum change type to nullable

[JefStat]

Encountered a bug on production due to this response data. Note the production cluster is running Elasticsearch 6.8

{
          "key" : "Morning 0",
          "doc_count" : 17,
          "subStatsAgg" : {
            "meta" : {
              "aggType" : "Sum"
            },
            "count" : 0,
            "min" : null,
            "max" : null,
            "avg" : null,
            "sum" : null
          }
        }

[russcam]

@JefStat which version of the client are you using against Elasticsearch 6.8.0?

Stats aggregate was changed in 7.0.0-beta1 of Elasticsearch to return sum of 0 when the count is 0 in elastic/elasticsearch#27193, so the client was updated in 4f64167 to reflect this in master and 7.x. The 6.x client still treats it as double?:

elasticsearch-net/src/Nest/Aggregations/AggregateJsonConverter.cs

Line 323 in f00e0e8

var sum = reader.Value as double?;

Are you using a 7.x client with 6.8, by chance?

[JefStat]

Are you using a 7.x client with 6.8, by chance?

Yes, we are. This is a temporary state before doing a rolling upgrade on Elasticsearch. I tested running the NEST 6.8 client on 7.3 Cluster and it did not work on several issues.

[JefStat]

I see a blog post has been released to partly address our issue https://www.elastic.co/blog/nest-and-elasticsearch-net-upgrading-your-codebase I can extrapolate from there