Fix packs generator error when server_bundle_js_file is empty

Add safety check to prevent PacksGenerator from attempting operations
on invalid file paths when server_bundle_js_file is not configured.

The configuration defaults to empty string when server-side rendering
is not enabled, which would cause the method to construct malformed
file paths and potentially fail.

This change follows the existing pattern used elsewhere in the class
where server_bundle_js_file.present? is checked before operations.

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/react_on_rails/packs_generator.rb

@@ -164,6 +164,7 @@ def generated_server_pack_file_content
 
     def add_generated_pack_to_server_bundle
       return if ReactOnRails.configuration.make_generated_server_bundle_the_entrypoint
+      return if ReactOnRails.configuration.server_bundle_js_file.empty?
 
       relative_path_to_generated_server_bundle = relative_path(server_bundle_entrypoint,
                                                                generated_server_bundle_file_path)