Fix GoogleEnterpriseSearchRetriever (langchain-ai#10546)

Replace this entire comment with:
- Description: fixed Google Enterprise Search Retriever where it was
consistently returning empty results,
- Issue: related to [issue
8219](langchain-ai#8219),
  - Dependencies: no dependencies,
  - Tag maintainer: @hwchase17 ,
  - Twitter handle: [Tomas Piaggio](https://twitter.com/TomasPiaggio)!

libs/langchain/langchain/retrievers/google_cloud_enterprise_search.py

@@ -156,7 +156,10 @@ def _convert_unstructured_search_response(
                 else "extractive_segments"
             )
 
-            for chunk in derived_struct_data.get(chunk_type, []):
+            if chunk_type not in derived_struct_data:
+                continue
+
+            for chunk in derived_struct_data[chunk_type]:
                 doc_metadata["source"] = derived_struct_data.get("link", "")
 
                 if chunk_type == "extractive_answers":