Return valid empty JSON response when no recovery information

This is a fix to send back to the client a valid empty JSON response in
the case when we have no recovery information.

Closes #5743

src/main/java/org/elasticsearch/action/admin/indices/recovery/RecoveryResponse.java

@@ -89,7 +89,7 @@ public class RecoveryResponse extends BroadcastOperationResponse implements ToXC
 
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
-
+        if (hasRecoveries()) {
             for (String index : shardResponses.keySet()) {
                 List<ShardRecoveryResponse> responses = shardResponses.get(index);
                 if (responses == null || responses.size() == 0) {
@@ -106,6 +106,7 @@ public class RecoveryResponse extends BroadcastOperationResponse implements ToXC
                 builder.endArray();
                 builder.endObject();
             }
+        }
         return builder;
     }
 

src/main/java/org/elasticsearch/rest/action/admin/indices/recovery/RestRecoveryAction.java

@@ -57,12 +57,10 @@ public class RestRecoveryAction extends BaseRestHandler {
         client.admin().indices().recoveries(recoveryRequest, new RestBuilderListener<RecoveryResponse>(channel) {
             @Override
             public RestResponse buildResponse(RecoveryResponse response, XContentBuilder builder) throws Exception {
-                if (response.hasRecoveries()) {
                 response.detailed(recoveryRequest.detailed());
                 builder.startObject();
                 response.toXContent(builder, request);
                 builder.endObject();
-                }
                 return new BytesRestResponse(OK, builder);
             }
         });