nixos/timesyncd: allow null for option servers

This gives the ability to not write `NTP=` to the `timesyncd.conf` file
(servers = null) as opposed to writing `NTP=` (servers = []) which is
interpreted slightly differently by systemd:

> When the empty string is assigned, the list of NTP servers is reset,
and all prior assignments will have no effect.

nixos/modules/system/boot/timesyncd.nix

@@ -21,12 +21,15 @@ in
       servers = mkOption {
         default = config.networking.timeServers;
         defaultText = literalExpression "config.networking.timeServers";
-        type = listOf str;
+        type = nullOr (listOf str);
         description = ''
           The set of NTP servers from which to synchronise.
-          Note if this is set to an empty list, the defaults systemd itself is
-          compiled with ({0..4}.nixos.pool.ntp.org) apply,
-          In case you want to disable timesyncd altogether, use the `enable` option.
+
+          Setting this option to an empty list will write `NTP=` to the
+          `timesyncd.conf` file as opposed to setting this option to null which
+          will remove `NTP=` entirely.
+
+          See man:timesyncd.conf(5) for details.
         '';
       };
       extraConfig = mkOption {
@@ -85,9 +88,11 @@ in
 
     environment.etc."systemd/timesyncd.conf".text = ''
       [Time]
+    ''
+    + optionalString (cfg.servers != null) ''
       NTP=${concatStringsSep " " cfg.servers}
-      ${cfg.extraConfig}
-    '';
+    ''
+    + cfg.extraConfig;
 
     users.users.systemd-timesync = {
       uid = config.ids.uids.systemd-timesync;