Fix AOOBE when setting min_doc_count to 0 in significant_terms

This commit fixes the computation of the subset size on empty buckets (doc count of 0).
The aggregator test refactoring in elastic#60683 revealed this bug.

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/GlobalOrdinalsStringTermsAggregator.java

@@ -561,7 +561,7 @@ private InternalAggregation[] buildAggregations(long[] owningBucketOrds) throws
  BucketUpdater<TB> updater = bucketUpdater(owningBucketOrds[ordIdx]);
  collectionStrategy.forEach(owningBucketOrds[ordIdx], new BucketInfoConsumer() {
  TB spare = null;
- 
+
  @Override
  public void accept(long globalOrd, long bucketOrd, long docCount) throws IOException {
  otherDocCount[finalOrdIdx] += docCount;
@@ -574,7 +574,7 @@ public void accept(long globalOrd, long bucketOrd, long docCount) throws IOExcep
  }
  }
  });
- 
+
  // Get the top buckets
  topBucketsPreOrd[ordIdx] = buildBuckets(ordered.size());
  for (int i = ordered.size() - 1; i >= 0; --i) {
@@ -799,7 +799,8 @@ SignificantStringTerms.Bucket buildEmptyTemporaryBucket() {
 
  @Override
  BucketUpdater<SignificantStringTerms.Bucket> bucketUpdater(long owningBucketOrd) throws IOException {
- long subsetSize = subsetSizes.get(owningBucketOrd);
+ // the subset size is missing for empty buckets (doc count of 0)
+ long subsetSize = owningBucketOrd < subsetSizes.size() ? subsetSizes.get(owningBucketOrd) : 0;
  return (spare, globalOrd, bucketOrd, docCount) -> {
  spare.bucketOrd = bucketOrd;
  oversizedCopy(lookupGlobalOrd.apply(globalOrd), spare.termBytes);
@@ -833,13 +834,15 @@ void buildSubAggs(SignificantStringTerms.Bucket[][] topBucketsPreOrd) throws IOE
 
  @Override
  SignificantStringTerms buildResult(long owningBucketOrd, long otherDocCount, SignificantStringTerms.Bucket[] topBuckets) {
+ // the subset size is missing for empty buckets (doc count of 0)
+ long subsetSize = owningBucketOrd < subsetSizes.size() ? subsetSizes.get(owningBucketOrd) : 0;
  return new SignificantStringTerms(
  name,
  bucketCountThresholds.getRequiredSize(),
  bucketCountThresholds.getMinDocCount(),
  metadata(),
  format,
- subsetSizes.get(owningBucketOrd),
+ subsetSize,
  supersetSize,
  significanceHeuristic,
  Arrays.asList(topBuckets)