Fixed NullPointerException on bulk request (#88358)

Java's `ArrayList.toArray()` returns provided array when collection is empty.
Here is created a one-element array which contains null element.
Thus, returned `BulkResponse` may contains a null element as `BulkItemResponse`.

Author: catap

docs/changelog/88358.yaml

@@ -0,0 +1,5 @@
+pr: 88385
+summary: Fixed NullPointerException on bulk request
+area: Distributed
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/action/bulk/Retry.java

@@ -195,7 +195,7 @@ private void addResponses(BulkResponse response, Predicate<BulkItemResponse> fil
         private BulkResponse getAccumulatedResponse() {
             BulkItemResponse[] itemResponses;
             synchronized (responses) {
-                itemResponses = responses.toArray(new BulkItemResponse[1]);
+                itemResponses = responses.toArray(new BulkItemResponse[0]);
             }
             long stopTimestamp = System.nanoTime();
             long totalLatencyMs = TimeValue.timeValueNanos(stopTimestamp - startTimestampNanos).millis();