Update RRF retriever to return an error when the fields param is an empty array

Author: Mikep86

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

@@ -115,7 +115,7 @@ public RRFRetrieverBuilder(
     ) {
         // Use a mutable list for childRetrievers so that we can use addChild
         super(childRetrievers == null ? new ArrayList<>() : new ArrayList<>(childRetrievers), rankWindowSize);
-        this.fields = fields == null ? List.of() : List.copyOf(fields);
+        this.fields = fields == null ? null : List.copyOf(fields);
         this.query = query;
         this.rankConstant = rankConstant;
     }
@@ -293,7 +293,7 @@ public void doToXContent(XContentBuilder builder, Params params) throws IOExcept
             builder.endArray();
         }
 
-        if (fields.isEmpty() == false) {
+        if (fields != null) {
             builder.startArray(FIELDS_FIELD.getPreferredName());
             for (String field : fields) {
                 builder.value(field);