sum of empty histogram is now null (#138378)

Part of #137988
Implement #138349 for the t-digest field type.

This changes the behavior of the sum sub-field of t-digest when the digest is empty. Prior to this PR we treated it as 0, and this changes it to be null. This avoids a division by zero error in ESQL when trying to calculate the average of an empty histogram. We are adopting the same behavior for the exponential histogram field (see PR linked above), and this is important to keep the semantics of the two fields as close as possible.

Author: not-napoleon

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: [ ]