Add test to ensure `end_time: null` is not dumped

tests/test_cli.py

@@ -46,6 +46,11 @@ def yaml_sink(_mocker, command: str):
         assert isinstance(cfg, Config)
 
     yaml_dump = yaml.dump(cfg)
+
+    # YAML Representer.ignore_aliases() should return True for Enums.
+    # If it returns True for other types, "null" is dumped explicitly, which is ugly.
+    assert "end_time: null" not in yaml_dump
+
     cfg_round_trip = yaml.load(yaml_dump)
     assert cfg_round_trip == cfg, yaml_dump