Improvements based on PR comments

Author: mridula-s109

x-pack/plugin/search-business-rules/src/main/java/org/elasticsearch/xpack/searchbusinessrules/retriever/PinnedRetrieverBuilder.java

@@ -160,12 +160,12 @@ public int rankWindowSize() {
      * Prioritizes docs over ids if both are present.
      *
      * @param baseQuery the base query to pin documents to
-     * @return a PinnedQueryBuilder or the original query if no pinned documents
+     * @return a PinnedQueryBuilder
+     * @throws IllegalArgumentException if baseQuery is null
      */
     private QueryBuilder createPinnedQuery(QueryBuilder baseQuery) {
-        if (baseQuery == null) {
-            baseQuery = new MatchAllQueryBuilder();
-        }
+        Objects.requireNonNull(baseQuery, "Underlying query cannot be null for pinned retriever");
+
         if (docs.isEmpty() == false) {
             return new PinnedQueryBuilder(baseQuery, docs.toArray(new SpecifiedDocument[0]));
         }

x-pack/plugin/search-business-rules/src/test/java/org/elasticsearch/xpack/searchbusinessrules/retriever/PinnedRetrieverBuilderTests.java

@@ -24,6 +24,7 @@
 import org.elasticsearch.xcontent.json.JsonXContent;
 import org.elasticsearch.xpack.searchbusinessrules.SpecifiedDocument;
 
+
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;

x-pack/plugin/search-business-rules/src/yamlRestTest/resources/rest-api-spec/test/search-business-rules/10_pinned_retriever.yml

@@ -333,11 +333,15 @@ setup:
   - gt: { hits.hits.0._score: 1.0 }
   - match:
       hits.hits.0._explanation.description: "Pinned document by ids, original explanation:"
+  - match:
+      hits.hits.0._explanation.details.0.description: "doc [0] with an original score of [1.7014124E38] is at rank [1] from the following source queries."
 
   - match: { hits.hits.1._id: doc2 }
   - gt: { hits.hits.1._score: 1.0 }
   - match:
       hits.hits.1._explanation.description: "Pinned document by ids, original explanation:"
+  - match:
+      hits.hits.1._explanation.details.0.description: "doc [1] with an original score of [1.7014122E38] is at rank [2] from the following source queries."
 
 ---
 "pinned retriever with empty parameters":
@@ -439,3 +443,16 @@ setup:
                   query:
                     match: { text: "document" }
                   sort: [ { "_id": "desc" } ]
+
+---
+"pinned retriever error case - null inner query":
+  - do:
+      catch: /Underlying query cannot be null for pinned retriever/
+      search:
+        index: test-index1
+        body:
+          retriever:
+            pinned:
+              ids: ["doc1"]
+              retriever:
+                standard: {}