fix: high level list method

Author: jaycee-li

google/cloud/aiplatform/base.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2020 Google LLC
+# Copyright 2022 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -1064,9 +1064,11 @@ def _list(
             or initializer.global_config.common_location_path(
                 project=project, location=location
             ),
-            "filter": filter,
         }
 
+        if filter:
+            list_request["filter"] = filter
+
         if order_by:
             list_request["order_by"] = order_by