userborn check causes error if unset

[This change](https://github.com/ryantm/agenix/commit/d80d1febd35ccf73455148a73bbc406d52dffc57), adding a check for `config.services.userborn.enable`, gave me an `attribute 'userborn' missing` error while trying to use agenix. I don't know my way round nix very well, so I'm not sure whether it's reasonable to expect this to always be set, but changing to the previous commit fixed my issue. I'm guessing there's a way to handle missing attributes more gracefully and still keep the check if it is present, but I'm not familiar. I'm on Arch, deploying to a NixOS AWS server if that helps. My non-working configuration is here: https://github.com/ratherforky/lectorial-tools/blob/04d43a707c07f7296f3a7e3897bfd8296db40a6f/flake.nix And this change (specifying the commit before the added userborn check) fixed it: https://github.com/ratherforky/lectorial-tools/commit/1808e3975ed6292a1ecb5914c60fb2bb1deb6826