Fix image_id absence issue (#3735)

### What problem does this PR solve?

#3731

### Type of change

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

api/apps/sdk/session.py

@@ -413,7 +413,7 @@ def list_session(chat_id,tenant_id):
                                 "document_id": chunk["doc_id"],
                                 "document_name": chunk["docnm_kwd"],
                                 "dataset_id": chunk["kb_id"],
-                                "image_id": chunk["image_id"],
+                                "image_id": chunk.get("image_id", ""),
                                 "similarity": chunk["similarity"],
                                 "vector_similarity": chunk["vector_similarity"],
                                 "term_similarity": chunk["term_similarity"],