Fix has_actions_for_brains check (#3087)

ml-agents/mlagents/trainers/env_manager.py

     brain_name_to_action_info: Dict[str, ActionInfo]
 
     def has_actions_for_brain(self, brain_name: str) -> bool:
-        return (
-            brain_name in self.brain_name_to_action_info
-            and self.brain_name_to_action_info[brain_name].outputs is not None
+        return brain_name in self.brain_name_to_action_info and bool(
+            self.brain_name_to_action_info[brain_name].outputs
         )