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

@@ -165,27 +165,24 @@ 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:
-            for kb_id in ids:
-                kbs = KnowledgebaseService.accessible(kb_id=kb_id, user_id=tenant_id)
-                if not kbs:
-                    return get_error_data_result(f"You don't own the dataset {kb_id}")
-                kbs = KnowledgebaseService.query(id=kb_id)
-                kb = kbs[0]
-                if kb.chunk_num == 0:
-                    return get_error_data_result(f"The dataset {kb_id} doesn't own parsed file")
-                
-            kbs = KnowledgebaseService.get_by_ids(ids)
-            embd_ids = [TenantLLMService.split_model_name_and_factory(kb.embd_id)[0] for kb in kbs]  # remove vendor suffix for comparison
-            embd_count = list(set(embd_ids))
-            if len(embd_count) != 1:
-                return get_result(
-                    message='Datasets use different embedding models."',
-                    code=settings.RetCode.AUTHENTICATION_ERROR)
-            req["kb_ids"] = 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:
+                return get_error_data_result(f"You don't own the dataset {kb_id}")
+            kbs = KnowledgebaseService.query(id=kb_id)
+            kb = kbs[0]
+            if kb.chunk_num == 0:
+                return get_error_data_result(f"The dataset {kb_id} doesn't own parsed file")
+            
+        kbs = KnowledgebaseService.get_by_ids(ids)
+        embd_ids = [TenantLLMService.split_model_name_and_factory(kb.embd_id)[0] for kb in kbs]  # remove vendor suffix for comparison
+        embd_count = list(set(embd_ids))
+        if len(embd_count) != 1:
+            return get_result(
+                message='Datasets use different embedding models."',
+                code=settings.RetCode.AUTHENTICATION_ERROR)
+        req["kb_ids"] = ids
     llm = req.get("llm")
     if llm:
         if "model_name" in llm: