nixos/pinchflat: make TZ variable optional (#452647)

nixos/modules/services/misc/pinchflat.nix

@@ -147,7 +147,6 @@ in
         StateDirectory = baseNameOf stateDir;
         Environment = [
           "PORT=${builtins.toString cfg.port}"
-          "TZ=${config.time.timeZone}"
           "MEDIA_PATH=${cfg.mediaDir}"
           "CONFIG_PATH=${stateDir}"
           "DATABASE_PATH=${stateDir}/db/pinchflat.db"
@@ -160,6 +159,7 @@ in
           "PHX_SERVER=true"
         ]
         ++ optional cfg.selfhosted [ "RUN_CONTEXT=selfhosted" ]
+        ++ optional (!isNull config.time.timeZone) "TZ=${config.time.timeZone}"
         ++ attrValues (mapAttrs (name: value: name + "=" + builtins.toString value) cfg.extraConfig);
         EnvironmentFile = optional (cfg.secretsFile != null) cfg.secretsFile;
         ExecStartPre = "${lib.getExe' cfg.package "migrate"}";