Fix AOOBE when setting min_doc_count to 0 in significant_terms (#60823)

This commit fixes the computation of the subset size on empty buckets (doc count of 0).
The aggregator test refactoring in #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) {
@@ -797,9 +797,14 @@ SignificantStringTerms.Bucket buildEmptyTemporaryBucket() {
  return new SignificantStringTerms.Bucket(new BytesRef(), 0, 0, 0, 0, null, format, 0);
  }
 
+ private long subsetSize(long owningBucketOrd) {
+ // if the owningBucketOrd is not in the array that means the bucket is empty so the size has to be 0
+ return owningBucketOrd < subsetSizes.size() ? subsetSizes.get(owningBucketOrd) : 0;
+ }
+
  @Override
  BucketUpdater<SignificantStringTerms.Bucket> bucketUpdater(long owningBucketOrd) throws IOException {
- long subsetSize = subsetSizes.get(owningBucketOrd);
+ long subsetSize = subsetSize(owningBucketOrd);
  return (spare, globalOrd, bucketOrd, docCount) -> {
  spare.bucketOrd = bucketOrd;
  oversizedCopy(lookupGlobalOrd.apply(globalOrd), spare.termBytes);
@@ -839,7 +844,7 @@ SignificantStringTerms buildResult(long owningBucketOrd, long otherDocCount, Sig
  bucketCountThresholds.getMinDocCount(),
  metadata(),
  format,
- subsetSizes.get(owningBucketOrd),
+ subsetSize(owningBucketOrd),
  supersetSize,
  significanceHeuristic,
  Arrays.asList(topBuckets)