programs.khal: uncomment locale config (#4319)

While the locale options were declared, the weren't used in the
generation of the config file, because the locale submodule missed a
default, which failed the tests. I added an empty attribute set as a
default, which fixes the test, and works with the defaults in the
submodule options as expected.

modules/programs/khal.nix

@@ -152,6 +152,7 @@ in {
       description = ''
         khal locale settings. 
       '';
+      default = { };
     };
   };
 
@@ -161,7 +162,7 @@ in {
     xdg.configFile."khal/config".text = concatStringsSep "\n" ([ "[calendars]" ]
       ++ mapAttrsToList genCalendarStr khalAccounts ++ [
         (generators.toINI { } {
-          # locale = definedAttrs (cfg.locale // { _module = null; });
+          locale = definedAttrs (cfg.locale // { _module = null; });
 
           default = optionalAttrs (!isNull primaryAccount) {
             highlight_event_days = true;