Added RRF retriever YAML test

Author: Mikep86

x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/rrf/310_rrf_retriever_simplified.yml

@@ -359,6 +359,18 @@ setup:
 
   - contains: { error.root_cause.0.reason: "[rrf] [query] cannot be empty" }
 
+  - do:
+      catch: bad_request
+      search:
+        index: test-index
+        body:
+          retriever:
+            rrf:
+              fields: [ ]
+              query: "foo"
+
+  - contains: { error.root_cause.0.reason: "[rrf] [fields] cannot be empty" }
+
   - do:
       catch: bad_request
       search: