fix: ordering terms aggregation on top metrics null values (#85774)

If top metrics buckets are never collected (i.e. a filter never
passing data to a top metrics aggregator) ordering results in an
index out of bounds exception. THis is because the data array
expected to include data is actually empty.

Here we use the special Double.NaN value to report missing data
to sort on for the top metrics metric field. This results in the
comparator ordering data using another comparator (the default
'_key' comprator).

docs/changelog/85774.yaml

@@ -0,0 +1,5 @@
+pr: 85774
+summary: "Fix: ordering terms aggregation on top metrics null values"
+area: Aggregations
+type: bug
+issues: []

x-pack/plugin/analytics/src/main/java/org/elasticsearch/xpack/analytics/topmetrics/TopMetricsAggregator.java

@@ -293,6 +293,9 @@ static class DoubleMetricValues extends CollectingMetricValues {
 
         @Override
         public double doubleValue(long index) {
+            if (index < 0 || index >= values.size()) {
+                return Double.NaN;
+            }
             return values.get(index);
         }
 
@@ -357,7 +360,7 @@ static class LongMetricValues extends CollectingMetricValues {
 
         @Override
         public double doubleValue(long index) {
-            if (empty.isEmpty(index)) {
+            if (empty.isEmpty(index) || index < 0 || index >= values.size()) {
                 return Double.NaN;
             }
             return values.get(index);