fix: keyerror issue (#3512)

### What problem does this PR solve?

#3511

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/sdk/session.py

                     "document_id": chunk["doc_id"],
                     "document_name": chunk["docnm_kwd"],
                     "dataset_id": chunk["kb_id"],
-                    "image_id": chunk["img_id"],
+                    "image_id": chunk.get("img_id", ""),
                     "similarity": chunk["similarity"],
                     "vector_similarity": chunk["vector_similarity"],
                     "term_similarity": chunk["term_similarity"],
-                    "positions": chunk["positions"],
+                    "positions": chunk.get("positions", []),
                 }
                 chunk_list.append(new_chunk)
             reference["chunks"] = chunk_list