Fix BWC error on pipeline aggs

I derped out on a last minute bug fix when backporting elastic#54282 and it
only causes the tests to fail about half the time. So I didn't catch
it until after merging. Great! This fixes it.

Author: nik9000

server/src/main/java/org/elasticsearch/search/aggregations/InternalAggregations.java

@@ -24,8 +24,8 @@
 import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.search.aggregations.InternalAggregation.ReduceContext;
 import org.elasticsearch.search.aggregations.pipeline.PipelineAggregator;
-import org.elasticsearch.search.aggregations.pipeline.SiblingPipelineAggregator;
 import org.elasticsearch.search.aggregations.pipeline.PipelineAggregator.PipelineTree;
+import org.elasticsearch.search.aggregations.pipeline.SiblingPipelineAggregator;
 import org.elasticsearch.search.aggregations.support.AggregationPath;
 
 import java.io.IOException;
@@ -114,7 +114,7 @@ public void writeTo(StreamOutput out) throws IOException {
                 mergePipelineTreeForBWCSerialization(pipelineTree);
                 out.writeNamedWriteableList(getInternalAggregations());
                 if (out.getVersion().onOrAfter(Version.V_6_7_0)) {
-                    out.writeNamedWriteableList(emptyList());
+                    out.writeNamedWriteableList(pipelineTree.aggregators());
                 }
             }
         } else {