Add missing require for compact_blank in configuration.rb

This adds the missing `require "active_support/core_ext/enumerable"` statement
that was mentioned in the PR description but was not included in the previous commit.

Without this require statement, the code would fail on Rails 5.2-6.0 because
compact_blank was only introduced in Rails 6.1. This gem supports Rails >= 5.2
(per react_on_rails.gemspec), so the require is necessary to ensure the method
is available on older Rails versions.

The compact_blank method is needed (rather than just compact) because:
- server_bundle_js_file defaults to "" (empty string)
- Pro config files could potentially be set to empty strings
- compact_blank filters out both nil and empty/blank values

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

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

Author: justin808

lib/react_on_rails/configuration.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/core_ext/enumerable"
+
 # rubocop:disable Metrics/ClassLength
 
 module ReactOnRails