secrets_dir, certfile and certkey were typed FilePath/DirectoryPath
with a None default, and several str fields (jwt_secret_key,
iourc_path, resources_path, default_nat_interface, vboxmanage_path,
vmrun_path) had the same mismatch. Serializing the unset values
emitted PydanticSerializationUnexpectedValue warnings on every
GET /v3/settings (two lines per request with SSL disabled).