Cardinality Aggregation dynamic pruning NPE catch (opensearch-project#17775)

* Update CardinalityAggregator.java

Signed-off-by: ajleong623 <[email protected]>

* Signed-off-by: Anthony Leong <[email protected]>

Signed-off-by: ajleong623 <[email protected]>

* Update CardinalityAggregator.java

Signed-off-by: ajleong623 <[email protected]>

* Checked Cardinality Aggregation dynamic pruning null pointer exception Signed-off-by: Anthony Leong <[email protected]>

Signed-off-by: ajleong623 <[email protected]>

---------

Signed-off-by: ajleong623 <[email protected]>
Signed-off-by: Sriram Ganesh <[email protected]>

Author: ajleong623

server/src/main/java/org/opensearch/search/aggregations/metrics/CardinalityAggregator.java

@@ -381,7 +381,13 @@ public void collect(int doc, long owningBucketOrd) throws IOException {
          * Note: the queue may be empty or the queue top may be null after pruning
          */
         private void prune(int doc) {
+            if (queue.size() == 0) {
+                return;
+            }
             DisiWrapper top = queue.top();
+            if (top == null) {
+                return;
+            }
             int curTopDoc = top.doc;
             if (curTopDoc == doc) {
                 do {