engine: fix NRE in TranspilerServer

Regressed in 000294e (fuse-open#482)

Author: mortend

src/tool/engine/FuseJS/TranspilerServer.cs

@@ -28,9 +28,11 @@ public class TranspilerServer : LogObject, ITranspiler
         bool _disposed;
         bool _isFaulted;
 
-        public TranspilerServer(Log log, UnoConfig config)
+        public TranspilerServer(Log log, UnoConfig config = null)
             : base(log)
         {
+            config ??= UnoConfig.Current;
+
             var script = Path.Combine(
                 config.GetNodeModuleDirectory("@fuse-open/transpiler"),
                 "server.min.js");