pipewire: Never set an empty LD_LIBRARY_PATH

An empty LD_LIBRARY_PATH may confuse some applications into appending
:, creating an empty segment that insecurely refers to the current
directory, not the absence of directories.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

nixos/modules/services/desktops/pipewire/pipewire.nix

@@ -239,7 +239,7 @@ in {
     };
 
     environment.sessionVariables.LD_LIBRARY_PATH =
-      lib.optional cfg.jack.enable "${cfg.package.jack}/lib";
+      lib.mkIf cfg.jack.enable [ "${cfg.package.jack}/lib" ];
 
     users = lib.mkIf cfg.systemWide {
       users.pipewire = {