They can be caught with nixos-option -r on an empty ({...}:{}) NixOS
configuration with warning like:

hardware.fancontrol.config = «error: The option value `hardware.fancontrol.config' in `/path/to/nixos/modules/services/hardware/fancontrol.nix' is not of type `strings concatenated with "\n"'.»;

They are actually the only kind of type error I get when using an empty configuration (not that bad!).

Obviously, most of the corrected options work well with null value, their implementation performs the check.
An indirect one is hardware.parallels.package which defaults to config.boot.kernelPackages.prl-tools. prl-tools is defined as null in all-packages.nix if kernel version is older than 4.15.

Motivation for this change

Correct options types, remove warnings.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.