Apply suggestions from code review

Co-authored-by: Leandro Lucarella <luca-frequenz@llucax.com>
Signed-off-by: Mathias L. Baumann <Marenz@users.noreply.github.com>

Author: Marenz

tests/test_managing_actor.py

@@ -347,7 +347,7 @@ def __init__(
             stream_timeout: timedelta,
         ) -> None:
             assert server_url
-            assert key is None or auth_key is None
+            assert key or auth_key
             assert call_timeout
             assert stream_timeout
             super().__init__()