Fix dx bundle fails when no assets are copied (#3519)

packages/cli/src/cli/bundle.rs

@@ -179,6 +179,7 @@ impl Bundle {
         }
 
         let asset_dir = bundle.build.asset_dir();
+        if asset_dir.exists() {
             let asset_dir_entries = std::fs::read_dir(&asset_dir)
                 .with_context(|| format!("failed to read asset directory {:?}", asset_dir))?;
             for entry in asset_dir_entries.flatten() {
@@ -195,6 +196,7 @@ impl Bundle {
                     .expect("to be set")
                     .insert(old.display().to_string(), new.display().to_string());
             }
+        }
 
         for resource_path in bundle_settings.resources.take().into_iter().flatten() {
             bundle_settings