cli: fix plugin serve file watching

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/beige-moose-look.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Only serve static assets if there is a public folder during `app:serve` and `plugin:serve`. This fixes a common bug that would break `plugin:serve` with an `EBUSY` error.

packages/cli/src/lib/bundler/server.ts

@@ -50,10 +50,12 @@ export async function serveBundle(options: ServeOptions) {
         publicPath: config.output?.publicPath as string,
         stats: 'errors-warnings',
       },
-      static: {
-        publicPath: config.output?.publicPath as string,
-        directory: paths.targetPublic ?? '/',
-      },
+      static: paths.targetPublic
+        ? {
+            publicPath: config.output?.publicPath as string,
+            directory: paths.targetPublic,
+          }
+        : undefined,
       historyApiFallback: {
         // Paths with dots should still use the history fallback.
         // See https://github.com/facebookincubator/create-react-app/issues/387.