[Bug Fix] Fix SelectValue placeholder fallback

[djalmaaraujo]

Summary

• Render the SelectValue placeholder when a provided block returns nil
• Add regression coverage for the nil-block placeholder case

Closes #292

Testing

• docker run --rm -v /Users/djalmaaraujo/dev/linkana/ruby_ui-issue-292:/workspace -w /workspace/gem ruby:3.4.7 bash -lc 'bundle install && bundle exec rake test TEST=test/ruby_ui/select_test.rb'