fix compilation warning

Author: kimchy

modules/elasticsearch/src/main/java/org/elasticsearch/index/mapper/json/JsonMapper.java

@@ -33,6 +33,8 @@
 @ThreadSafe
 public interface JsonMapper extends ToJson {
 
+    public static final JsonMapper[] EMPTY_ARRAY = new JsonMapper[0];
+
     @NotThreadSafe
     public static class BuilderContext {
         private final JsonPath jsonPath;

modules/elasticsearch/src/main/java/org/elasticsearch/index/mapper/json/JsonObjectMapper.java

@@ -411,7 +411,7 @@ private void serializeValue(JsonParseContext jsonContext, String currentFieldNam
     }
 
     @Override public void toJson(JsonBuilder builder, Params params) throws IOException {
-        toJson(builder, params, null);
+        toJson(builder, params, JsonMapper.EMPTY_ARRAY);
     }
 
     public void toJson(JsonBuilder builder, Params params, JsonMapper... additionalMappers) throws IOException {