Sum of empty t-digest should be null

x-pack/plugin/analytics/src/main/java/org/elasticsearch/xpack/analytics/mapper/TDigestFieldMapper.java

@@ -402,10 +402,13 @@ public void parse(DocumentParserContext context) throws IOException {
                 );
             }
             NumericDocValuesField countField = new NumericDocValuesField(valuesCountSubFieldName(fullPath()), parsedTDigest.count());
-            NumericDocValuesField sumField = new NumericDocValuesField(
-                valuesSumSubFieldName(fullPath()),
-                NumericUtils.doubleToSortableLong(parsedTDigest.sum())
-            );
+            NumericDocValuesField sumField = null;
+            if (Double.isNaN(parsedTDigest.sum()) == false) {
+                sumField = new NumericDocValuesField(
+                    valuesSumSubFieldName(fullPath()),
+                    NumericUtils.doubleToSortableLong(parsedTDigest.sum())
+                );
+            }
             if (context.doc().getByKey(fieldType().name()) != null) {
                 throw new IllegalArgumentException(
                     "Field ["
@@ -417,7 +420,9 @@ public void parse(DocumentParserContext context) throws IOException {
             }
             context.doc().addWithKey(fieldType().name(), digestField);
             context.doc().add(countField);
-            context.doc().add(sumField);
+            if (sumField != null) {
+                context.doc().add(sumField);
+            }
             if (maxField != null) {
                 context.doc().add(maxField);
             }
@@ -564,8 +569,12 @@ public DocValuesLoader docValuesLoader(LeafReader leafReader, int[] docIdsInLeaf
                         max = Double.NaN;
                     }
 
-                    sumValues.advanceExact(docId);
-                    sum = NumericUtils.sortableLongToDouble(sumValues.longValue());
+                    if (sumValues != null) {
+                        sumValues.advanceExact(docId);
+                        sum = NumericUtils.sortableLongToDouble(sumValues.longValue());
+                    } else {
+                        sum = Double.NaN;
+                    }
 
                     binaryValue = docValues.binaryValue();
                     return true;
@@ -595,7 +604,9 @@ public void write(XContentBuilder b) throws IOException {
             if (Double.isNaN(max) == false) {
                 b.field("max", max);
             }
-            b.field("sum", sum);
+            if (Double.isNaN(sum) == false) {
+                b.field("sum", sum);
+            }
 
             b.startArray(CENTROIDS_NAME);
             while (value.next()) {

x-pack/plugin/analytics/src/main/java/org/elasticsearch/xpack/analytics/mapper/TDigestParser.java

@@ -158,7 +158,7 @@ public static ParsedTDigest parse(String mappedFieldName, XContentParser parser)
             );
         }
         if (centroids.isEmpty()) {
-            sum = 0.0;
+            sum = null;
             min = null;
             max = null;
         }

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/analytics/t_digest_fieldtype.yml

@@ -276,7 +276,6 @@ TDigest with synthetic source and empty digest:
   - match:
       _source:
         latency:
-          sum: 0.0
           centroids: [ ]
           counts: [ ]