fix NPE occuring in ci

Author: mridula-s109

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

@@ -175,7 +175,11 @@ private QueryBuilder createPinnedQuery(QueryBuilder baseQuery) {
     @Override
     protected SearchSourceBuilder finalizeSourceBuilder(SearchSourceBuilder source) {
         validateSort(source);
-        source.query(createPinnedQuery(source.query()));
+        QueryBuilder underlyingQuery = source.query();
+        if (underlyingQuery == null) {
+            throw new IllegalArgumentException("[underlying query] must not be null for pinned retriever");
+        }
+        source.query(createPinnedQuery(underlyingQuery));
         return source;
     }