fix: normalize null list fields to [] in update_data before DB write

When a client sends null for assigned_team_ids or assigned_key_ids, ensure
the DB receives [] instead of null, preventing null from being stored where
empty list is expected. Extend test to verify the DB call uses [].

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/proxy/management_endpoints/access_group_endpoints.py

@@ -392,6 +392,8 @@ async def update_access_group(
     update_fields = data.model_dump(exclude_unset=True)
     update_data: dict = {"updated_by": user_api_key_dict.user_id}
     for field, value in update_fields.items():
+        if field in ("assigned_team_ids", "assigned_key_ids") and value is None:
+            value = []
         update_data[field] = value
 
     # Initialize delta lists before the try block so they remain accessible

tests/test_litellm/proxy/management_endpoints/test_access_group_endpoints.py

@@ -1169,7 +1169,7 @@ def test_delete_access_group_handles_out_of_sync_assigned_keys(client_and_mocks)
 
 
 def test_update_access_group_null_assigned_ids_treated_as_empty(client_and_mocks):
-    """Update with explicit null for assigned_*_ids clears the list without TypeError."""
+    """Update with explicit null for assigned_*_ids clears the list and writes [] to DB."""
     client, _, mock_table, *_ = client_and_mocks
 
     existing = _make_access_group_record(
@@ -1185,3 +1185,8 @@ def test_update_access_group_null_assigned_ids_treated_as_empty(client_and_mocks
         json={"assigned_team_ids": None, "assigned_key_ids": None},
     )
     assert resp.status_code == 200
+
+    # Verify the DB update was called with [] (not null) for list fields
+    update_call_kwargs = mock_table.update.call_args.kwargs
+    assert update_call_kwargs["data"]["assigned_team_ids"] == []
+    assert update_call_kwargs["data"]["assigned_key_ids"] == []