Make blank/empty comparisons invariant to ActiveSupport

- Implement liquid_blank? and liquid_empty? methods in Condition
  to emulate ActiveSupport's behavior when it's not loaded
- This ensures templates like `{% if x == blank %}` work identically
  whether ActiveSupport is loaded or not
- Update liquid-spec adapters for new API (ctx parameter)
- Add rake spec task for running liquid-spec matrix

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

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Rakefile

@@ -148,3 +148,9 @@ end
 task :console do
   exec 'irb -I lib -r liquid'
 end
+
+desc('run liquid-spec suite across all adapters')
+task :spec do
+  adapters = Dir['./spec/*.rb'].join(',')
+  sh "bundle exec liquid-spec matrix --adapters=#{adapters} --reference=ruby_liquid"
+end