nixos/timezone: Fix evaluation error

Evaluation error introduced in a0d464033c2bacffa9c05d27903fb03cd09ace46.

If the value for timeZone is null it shouldn't be even tried to coerce
it into a string.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @lheckemann, @joachifm

aszlig 4f901203 4ed291b3

Changed files
+1 -1
nixos
modules
config
+1 -1
nixos/modules/config/timezone.nix
···
environment.etc = {
zoneinfo.source = tzdir;
-
} // lib.optionalAttrs (config.time.timeZone == null) {
+
} // lib.optionalAttrs (config.time.timeZone != null) {
localtime.source = "/etc/zoneinfo/${config.time.timeZone}";
localtime.mode = "direct-symlink";
};