Properly size empty filters (#71864)

This saves a few ArrayList realocations by fixing an order of operations
bug issue in the "empty" handling arm for `filters`. I've also added a
test so I can debug that branch.

Author: nik9000

server/src/main/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregator.java

@@ -251,7 +251,7 @@ public InternalAggregation[] buildAggregations(long[] owningBucketOrds) throws I
     @Override
     public InternalAggregation buildEmptyAggregation() {
         InternalAggregations subAggs = buildEmptySubAggregations();
-        List<InternalFilters.InternalBucket> buckets = new ArrayList<>(filters.size() + otherBucketKey == null ? 0 : 1);
+        List<InternalFilters.InternalBucket> buckets = new ArrayList<>(filters.size() + (otherBucketKey == null ? 0 : 1));
         for (QueryToFilterAdapter<?> filter : filters) {
             InternalFilters.InternalBucket bucket = new InternalFilters.InternalBucket(filter.key().toString(), 0, subAggs, keyed);
             buckets.add(bucket);

server/src/test/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregatorTests.java

@@ -81,6 +81,7 @@
 import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.lessThan;
+import static org.hamcrest.Matchers.nullValue;
 import static org.mockito.Mockito.mock;
 
 public class FiltersAggregatorTests extends AggregatorTestCase {
@@ -115,6 +116,35 @@ public void testEmpty() throws Exception {
         directory.close();
     }
 
+    public void testBuildEmpty() throws IOException {
+        int numFilters = randomIntBetween(1, 10);
+        QueryBuilder[] filters = new QueryBuilder[numFilters];
+        for (int i = 0; i < filters.length; i++) {
+            filters[i] = QueryBuilders.termQuery("field", randomAlphaOfLength(5));
+        }
+        FiltersAggregationBuilder builder = new FiltersAggregationBuilder("test", filters);
+        boolean askForOtherBucket = true;
+        if (askForOtherBucket) {
+            builder.otherBucket(true).otherBucketKey("other");
+        }
+        withAggregator(
+            builder,
+            new MatchAllDocsQuery(),
+            iw -> {},
+            (searcher, aggregator) -> {
+                InternalFilters result = (InternalFilters) aggregator.buildEmptyAggregation();
+                for (int i = 0; i < filters.length; i++) {
+                    assertThat(result.getBucketByKey(String.valueOf(i)).getDocCount(), equalTo(0L));
+                }
+                if (askForOtherBucket) {
+                    assertThat(result.getBucketByKey("other").getDocCount(), equalTo(0L));
+                } else {
+                    assertThat(result.getBucketByKey("other"), nullValue());
+                }
+            }
+        );
+    }
+
     public void testNoFilters() throws IOException {
         testCase(new FiltersAggregationBuilder("test", new KeyedFilter[0]), new MatchAllDocsQuery(), iw -> {
             iw.addDocument(List.of());