Don't output empty ext object in SearchSourceBuilder#toXContent (#22093)

We shouldn't output an empty ext object if no ext sections have been set to the SearchSourceBuilder.

Closes #20969

core/src/main/java/org/elasticsearch/search/builder/SearchSourceBuilder.java

@@ -1228,7 +1228,7 @@ public final class SearchSourceBuilder extends ToXContentToBytes implements Writ
             builder.field(STATS_FIELD.getPreferredName(), stats);
         }
 
-        if (extBuilders != null) {
+        if (extBuilders != null && extBuilders.isEmpty() == false) {
             builder.startObject(EXT_FIELD.getPreferredName());
             for (SearchExtBuilder extBuilder : extBuilders) {
                 extBuilder.toXContent(builder, params);

core/src/test/java/org/elasticsearch/search/builder/SearchSourceBuilderTests.java

@@ -31,10 +31,12 @@ import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentFactory;
+import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.index.query.QueryBuilders;
 import org.elasticsearch.index.query.QueryParseContext;
+import org.elasticsearch.index.query.RandomQueryBuilder;
 import org.elasticsearch.search.AbstractSearchTestCase;
 import org.elasticsearch.search.rescore.QueryRescorerBuilder;
 import org.elasticsearch.search.sort.FieldSortBuilder;
@@ -44,6 +46,7 @@ import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.test.EqualsHashCodeTestUtils;
 
 import java.io.IOException;
+import java.util.Map;
 
 import static org.hamcrest.CoreMatchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
@@ -290,6 +293,29 @@ public class SearchSourceBuilderTests extends AbstractSearchTestCase {
         }
     }
 
+    public void testToXContent() throws  IOException {
+        //verify that only what is set gets printed out through toXContent
+        XContentType xContentType = randomFrom(XContentType.values());
+        {
+            SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder();
+            XContentBuilder builder = XContentFactory.contentBuilder(xContentType);
+            searchSourceBuilder.toXContent(builder, ToXContent.EMPTY_PARAMS);
+            BytesReference source = builder.bytes();
+            Map<String, Object> sourceAsMap = XContentHelper.convertToMap(source, false).v2();
+            assertEquals(0, sourceAsMap.size());
+        }
+        {
+            SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder();
+            searchSourceBuilder.query(RandomQueryBuilder.createQuery(random()));
+            XContentBuilder builder = XContentFactory.contentBuilder(xContentType);
+            searchSourceBuilder.toXContent(builder, ToXContent.EMPTY_PARAMS);
+            BytesReference source = builder.bytes();
+            Map<String, Object> sourceAsMap = XContentHelper.convertToMap(source, false).v2();
+            assertEquals(1, sourceAsMap.size());
+            assertEquals("query", sourceAsMap.keySet().iterator().next());
+        }
+    }
+
     public void testEmptyPostFilter() throws IOException {
         SearchSourceBuilder builder = new SearchSourceBuilder();
         String query = "{ \"post_filter\": {} }";