Add a sentinel to make sure Flow does not yet exist.

Author: PGijsbers

tests/test_flows/test_flow_functions.py

@@ -233,4 +233,5 @@ def test_sklearn_to_flow_list_of_lists(self):
         from sklearn.preprocessing import OrdinalEncoder
         ordinal_encoder = OrdinalEncoder(categories=[[0, 1], [0, 1]])
         flow = openml.flows.sklearn_to_flow(ordinal_encoder)
+        self._add_sentinel_to_flow_name(flow)
         flow.publish()