Fix packs generator error when server_bundle_js_file is empty (#1802)

* 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.

Author: justin808

CHANGELOG.md

@@ -23,6 +23,10 @@ After a release, please make sure to run `bundle exec rake update_changelog`. Th
 
 Changes since the last non-beta release.
 
+#### Bug Fixes
+
+- **Packs generator**: Fixed error when `server_bundle_js_file` configuration is empty (default). Added safety check to prevent attempting operations on invalid file paths when server-side rendering is not configured. [PR 1802](https://github.com/shakacode/react_on_rails/pull/1802)
+
 ### [16.0.1-rc.2] - 2025-09-20
 
 #### Bug Fixes

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)