[GSK-2079] Fix issue with filter and empty dataset (Giskard-AI#1558)

Fix issue with filter and empty dataset

Co-authored-by: Henrique Chaves <44180294+henchaves@users.noreply.github.com>
Co-authored-by: Inoki <weixuan@giskard.ai>

Author: 3 people

giskard/ml_worker/websocket/listener.py

@@ -814,7 +814,7 @@ def get_push_objects(client: Optional[GiskardClient], params: websocket.GetPushP
 
     # if df is empty, return early
     if df.empty:
-        return None, None, None, None
+        return None
 
     push_functions = {
         PushKind.CONTRIBUTION: create_contribution_push,