PostfixRelay: Fix a regression in Nix expression that caused a syntax…

[frlan]

… error while evaluation

Fixes

- In `/etc/local/nixos/postfix-relay.nix': "[xxx.fcio.net]:1025"
err>	
err>	error:
err>	       … while calling the 'head' builtin
err>	         at /nix/store/d50hvr97lw8i93lck08ypzsfnxlfcll7-source/nixpkgs/lib/attrsets.nix:1696:13:
err>	         1695|           if length values == 1 || pred here (elemAt values 1) (head values) then
err>	         1696|             head values
err>	             |             ^
err>	         1697|           else
err>	
err>	       … while evaluating the attribute 'value'
err>	         at /nix/store/d50hvr97lw8i93lck08ypzsfnxlfcll7-source/nixpkgs/lib/modules.nix:1118:7:
err>	         1117|     // {
err>	         1118|       value = addErrorContext "while evaluating the option `${showOption loc}':" value;
err>	             |       ^
err>	         1119|       inherit (res.defsFinal') highestPrio;
err>	
err>	       … while evaluating the option `system.build.toplevel':
err>	
err>	       … while evaluating definitions from `/nix/store/d50hvr97lw8i93lck08ypzsfnxlfcll7-source/nixpkgs/nixos/modules/system/activation/top-level.nix':
err>	
err>	       … while evaluating the option `warnings':
err>	
err>	       … while evaluating definitions from `/nix/store/d50hvr97lw8i93lck08ypzsfnxlfcll7-source/nixpkgs/nixos/modules/system/boot/systemd.nix':
err>	
err>	       … while evaluating the option `systemd.services.postfix-setup.serviceConfig':
err>	
err>	       … while evaluating definitions from `/nix/store/d50hvr97lw8i93lck08ypzsfnxlfcll7-source/nixpkgs/nixos/modules/system/boot/systemd.nix':
err>	
err>	       … while evaluating the option `systemd.services.postfix-setup.script':
err>	
err>	       … while evaluating definitions from `/nix/store/d50hvr97lw8i93lck08ypzsfnxlfcll7-source/nixpkgs/nixos/modules/services/mail/postfix.nix':
err>	
err>	       … while evaluating the option `services.postfix.settings.main.relayhost':
err>	
err>	       (stack trace truncated; use '--show-trace' to show the full, detailed trace)
err>	
err>	       error: A definition for option `services.postfix.settings.main.relayhost' is not of type `null or (list of string)'. Definition values:
err>	       - In `/etc/local/nixos/postfix-relay.nix': "[xxx.fcio.net]:1025"
err>