don't serialize unneeded filter

src/main/java/org/elasticsearch/index/query/functionscore/FunctionScoreQueryBuilder.java

@@ -22,7 +22,10 @@ package org.elasticsearch.index.query.functionscore;
 import org.elasticsearch.ElasticSearchException;
 import org.elasticsearch.common.lucene.search.function.CombineFunction;
 import org.elasticsearch.common.xcontent.XContentBuilder;
-import org.elasticsearch.index.query.*;
+import org.elasticsearch.index.query.BaseQueryBuilder;
+import org.elasticsearch.index.query.BoostableQueryBuilder;
+import org.elasticsearch.index.query.FilterBuilder;
+import org.elasticsearch.index.query.QueryBuilder;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -124,10 +127,8 @@ public class FunctionScoreQueryBuilder extends BaseQueryBuilder implements Boost
             builder.startArray("functions");
             for (int i = 0; i < filters.size(); i++) {
                 builder.startObject();
-                builder.field("filter");
-                if (filters.get(i) == null) {
-                    FilterBuilders.matchAllFilter().toXContent(builder, params);
-                } else {
+                if (filters.get(i) != null) {
+                    builder.field("filter");
                     filters.get(i).toXContent(builder, params);
                 }
                 scoreFunctions.get(i).toXContent(builder, params);