[Flaky test] ExtendedStatsAggregationUsageDocCountZeroTests returns null values

[russcam]

ExtendedStatsAggregationUsageDocCountZeroTests returns null values where numeric values are expected, which breaks deserialization:

{
  "took" : 51,
  "timed_out" : false,
  "_shards" : {
    "total" : 2,
    "successful" : 2,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 0,
      "relation" : "eq"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "extended_stats#commit_stats" : {
      "count" : 0,
      "min" : null,
      "max" : null,
      "avg" : null,
      "sum" : 0.0,
      "sum_of_squares" : null,
      "variance" : null,
      "variance_population" : null,
      "variance_sampling" : null,
      "std_deviation" : null,
      "std_deviation_population" : null,
      "std_deviation_sampling" : null,
      "std_deviation_bounds" : {
        "upper" : null,
        "lower" : null
      }
    }
  }
}

Investigate if this is a change in how extended stats are emitted

[russcam]

Looks like this is a result of new fields added

      "variance_population" : null,
      "variance_sampling" : null,

and

      "std_deviation_population" : null,
      "std_deviation_sampling" : null,

[russcam]

Related to elastic/elasticsearch#49782