nixos/syncthing: keep manually added dirs/devices

[raindev]

Description of changes

If folders and devices are not configured explicitly, do not wipe the changes done via the web GUI. Currently the list of devices or folders will be reset unless overrideFolders/overrideDevices is disabled.

Juding from #62157 this was the intention all along but the behaviour was changed unintentionally at a later point.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[ncfavier]

This disallows expressing that a machine should not have any configured devices, resp. folders, which I suppose is probably fine.

[raindev]

Thanks for having a look Naïm. I suppose it would be preferable to have overrideFolders and overrideDevices both false by default. This would allow to avoid the surprising behaviour of manually configured folders & devices disappearing while still allowing to enforce keeping the configured options, including empty. But changing the defaults would be backward incompatible of course.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/1343