Fix type issue

Author: PGijsbers

tests/database/flows_test.py

@@ -6,6 +6,7 @@
 
 def test_database_flow_exists(flow: Flow, expdb_test: Connection) -> None:
     retrieved_flow = database.flows.get_by_name(flow.name, flow.external_version, expdb_test)
+    assert retrieved_flow is not None
     assert retrieved_flow.id == flow.id
     # when using actual ORM, can instead ensure _all_ fields match.