fix:passing empty database array when updating assistant (#9570)

### What problem does this PR solve?

When the `dataset_ids` parameter is omitted in the **update assistant**
request, Passing an empty array `[]` triggers a misleading
message"Dataset use different embedding models", while omitting the
field does not.
To fix this, we:
- Provide a default empty list: `ids = req.get("dataset_ids", [])`.  
- Replace the `is not None` check with a truthy check: `if ids:`.

**Files changed**  
`api/apps/sdk/chat.py`  
- L153: `ids = req.get("dataset_ids")` → `ids = req.get("dataset_ids",
[])`
- L156: `if ids is not None:` → `if ids:`

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

api/apps/sdk/chat.py

@@ -150,10 +150,10 @@ def update(tenant_id, chat_id):
     if not DialogService.query(tenant_id=tenant_id, id=chat_id, status=StatusEnum.VALID.value):
         return get_error_data_result(message="You do not own the chat")
     req = request.json
-    ids = req.get("dataset_ids")
+    ids = req.get("dataset_ids", [])
     if "show_quotation" in req:
         req["do_refer"] = req.pop("show_quotation")
-    if ids is not None:
+    if ids:
         for kb_id in ids:
             kbs = KnowledgebaseService.accessible(kb_id=kb_id, user_id=tenant_id)
             if not kbs: