Empty weight shouldnt be serialised

Author: mridula-s109

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java

@@ -335,7 +335,7 @@ public void doToXContent(XContentBuilder builder, Params params) throws IOExcept
 
         builder.field(RANK_WINDOW_SIZE_FIELD.getPreferredName(), rankWindowSize);
         builder.field(RANK_CONSTANT_FIELD.getPreferredName(), rankConstant);
-        if (fields == null && query == null) {
+        if (weights.length > 0) {
             builder.startArray(WEIGHTS_FIELD.getPreferredName());
             for (float weight : weights) {
                 builder.value(weight);