when value is not a string

Motivation for this change

I was just hit by this because with the current master systemd version, networkConfig.MulticastDNS has changed from a bool to a three-state string, which resulted in an unspecific

error: cannot coerce a boolean to a string, at nixos/modules/system/boot/systemd-lib.nix:66:8

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Fits CONTRIBUTING.md.