Fix webpack config to fallback when privateOutputPath is undefined

The generated serverWebpackConfig.js was failing when Shakapacker 9+ was
detected but private_output_path wasn't configured in shakapacker.yml.
This caused config.privateOutputPath to be undefined, resulting in webpack
building to an incorrect location.

Changes:
- Add fallback to hardcoded path when config.privateOutputPath is undefined
- Ensures server bundle is always built to a valid location
- Maintains backward compatibility with all Shakapacker versions

This fixes the "server-bundle.js cannot be read" error in generator examples.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

lib/generators/react_on_rails/templates/base/base/config/webpack/serverWebpackConfig.js.tt

@@ -58,7 +58,7 @@ const configureServer = () => {
     globalObject: 'this',
     // If using the React on Rails Pro node server renderer, uncomment the next line
     // libraryTarget: 'commonjs2',
-    path: <%= shakapacker_version_9_or_higher? ? 'config.privateOutputPath' : "require('path').resolve(__dirname, '../../ssr-generated')" %>,
+    path: <%= shakapacker_version_9_or_higher? ? "config.privateOutputPath || require('path').resolve(__dirname, '../../ssr-generated')" : "require('path').resolve(__dirname, '../../ssr-generated')" %>,
     // No publicPath needed since server bundles are not served via web
     // https://webpack.js.org/configuration/output/#outputglobalobject
   };