fix: handle empty example text

[puneetdixit200]

Summary

• Return an empty line list when an @example body is nil or blank
• Add Cucumber coverage for titled empty examples and nil text normalization

Closes #15

Verification

• PATH="/opt/homebrew/opt/ruby/bin:$PATH" bundle exec cucumber features/yard_example_test.feature:283:298
• git diff --check
• PATH="/opt/homebrew/opt/ruby/bin:$PATH" bundle exec rake

AI disclosure

This contribution was prepared with AI assistance and manually reviewed before submission.