StackOverflowError when calling BulkRequest#add (elastic#41672)

Removing of payload in BulkRequest (elastic#39843) had a side effect of making
`BulkRequest.add(DocWriteRequest<?>...)` (with varargs) recursive, thus
leading to StackOverflowError. This PR adds a small change in
RequestConvertersTests to show the error and the corresponding fix in
`BulkRequest`.

Fixes elastic#41668

client/rest-high-level/src/test/java/org/elasticsearch/client/RequestConvertersTests.java

@@ -874,6 +874,7 @@ public void testBulk() throws IOException {
  XContentType xContentType = randomFrom(XContentType.JSON, XContentType.SMILE);
 
  int nbItems = randomIntBetween(10, 100);
+ DocWriteRequest<?>[] requests = new DocWriteRequest<?>[nbItems];
  for (int i = 0; i < nbItems; i++) {
  String index = randomAlphaOfLength(5);
  String id = randomAlphaOfLength(5);
@@ -913,8 +914,9 @@ public void testBulk() throws IOException {
  docWriteRequest.setIfSeqNo(randomNonNegativeLong());
  docWriteRequest.setIfPrimaryTerm(randomLongBetween(1, 200));
  }
- bulkRequest.add(docWriteRequest);
+ requests[i] = docWriteRequest;
  }
+ bulkRequest.add(requests);
 
  Request request = RequestConverters.bulk(bulkRequest);
  assertEquals("/_bulk", request.getEndpoint());

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

@@ -99,7 +99,7 @@ public BulkRequest(@Nullable String globalIndex, @Nullable String globalType) {
  */
  public BulkRequest add(DocWriteRequest<?>... requests) {
  for (DocWriteRequest<?> request : requests) {
- add(request, null);
+ add(request);
  }
  return this;
  }