discocss: only generate css when provided

No need to generate a file with empty contents.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/programs/discocss.nix

@@ -50,6 +50,8 @@ in
       ))
     ];
 
-    xdg.configFile."discocss/custom.css".text = cfg.css;
+    xdg.configFile."discocss/custom.css" = lib.mkIf (cfg.css != "") {
+      text = cfg.css;
+    };
   };
 }