Fix：validate knowledge base association before document upload (#5373)

### What problem does this PR solve?

fix this bug: https://github.com/infiniflow/ragflow/issues/5368

### Type of change

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

Co-authored-by: wenju.li <wenju.li@deepctr.cn>

api/db/services/document_service.py

@@ -504,6 +504,9 @@ def doc_upload_and_parse(conversation_id, file_objs, user_id):
     assert e, "Conversation not found!"
 
     e, dia = DialogService.get_by_id(conv.dialog_id)
+    if not dia.kb_ids:
+        raise LookupError("No knowledge base associated with this conversation. "
+                          "Please add a knowledge base before uploading documents")
     kb_id = dia.kb_ids[0]
     e, kb = KnowledgebaseService.get_by_id(kb_id)
     if not e: