Core: Fix IndicesSegmentResponse.toXcontent() serialization

When index sorting is enabled, toXContent tried to serialize an
SortField object, resulting in an exception, when using the _segments
endpoint.

Relates elastic#29120

server/src/main/java/org/elasticsearch/action/admin/indices/segments/IndicesSegmentResponse.java

@@ -186,7 +186,9 @@ private static void toXContent(XContentBuilder builder, Sort sort) throws IOExce
                 builder.field("mode", ((SortedSetSortField) field).getSelector()
                     .toString().toLowerCase(Locale.ROOT));
             }
-            builder.field("missing", field.getMissingValue());
+            if (field.getMissingValue() != null) {
+                builder.field("missing", field.getMissingValue().toString());
+            }
             builder.field("reverse", field.getReverse());
             builder.endObject();
         }

server/src/test/java/org/elasticsearch/action/admin/indices/segments/IndicesSegmentResponseTests.java

@@ -0,0 +1,53 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.indices.segments;
+
+import org.apache.lucene.search.Sort;
+import org.apache.lucene.search.SortField;
+import org.elasticsearch.cluster.routing.ShardRouting;
+import org.elasticsearch.cluster.routing.ShardRoutingState;
+import org.elasticsearch.cluster.routing.TestShardRouting;
+import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.index.engine.Segment;
+import org.elasticsearch.test.ESTestCase;
+
+import java.util.Collections;
+
+import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
+
+public class IndicesSegmentResponseTests extends ESTestCase {
+
+    public void testToXContentSerialiationWithSortedFields() throws Exception {
+        ShardRouting shardRouting = TestShardRouting.newShardRouting("foo", 0, "node_id", true, ShardRoutingState.STARTED);
+        Segment segment = new Segment("my");
+
+        SortField sortField = new SortField("foo", SortField.Type.STRING);
+        sortField.setMissingValue(SortField.STRING_LAST);
+        segment.segmentSort = new Sort(sortField);
+
+        ShardSegments shardSegments = new ShardSegments(shardRouting, Collections.singletonList(segment));
+        IndicesSegmentResponse response =
+            new IndicesSegmentResponse(new ShardSegments[] { shardSegments }, 1, 1, 0, Collections.emptyList());
+        try (XContentBuilder builder = jsonBuilder()) {
+            response.toXContent(builder, ToXContent.EMPTY_PARAMS);
+        }
+    }
+}