Fix:Update chat assistant with an empty dataset (#7354)

### What problem does this PR solve?

When updating a chat assistant using API,if the dataset attached by the
current chat assistant is not empty，setting dataset to
null("dataset_ids":[]) will cause update failure：'dataset_ids' can't be
empty

### Type of change

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

api/apps/sdk/chat.py

@@ -165,10 +165,7 @@ def update(tenant_id, chat_id):
     ids = req.get("dataset_ids")
     if "show_quotation" in req:
         req["do_refer"] = req.pop("show_quotation")
-    if "dataset_ids" in req:
-        if not ids:
-            return get_error_data_result("`dataset_ids` can't be empty")
-        if ids:
+    if ids is not None:
         for kb_id in ids:
             kbs = KnowledgebaseService.accessible(kb_id=kb_id, user_id=tenant_id)
             if not kbs: