fix (lightbox): possible php error if the attrs array is unavailable

[bfintal]

To test:

1. Post with Stackable blocks without lightbox functionality should not show any php warnings/errors
2. Post with lightbox functionality should not show any php warnings/errors

[github-actions[bot]]

🤖 Pull request artifacts

file	commit
pr3083-stackable-3083-merge.zip	60194724f87cb8f2a27b032582a8850f5d9ad017

[github-actions[bot]]

Try this Pull Request in the WordPress playground: https://playground.wordpress.net/?mode=seamless#{"landingPage":"/wp-admin/post-new.php?post_type=page","preferredVersions":{"php":"latest","wp":"latest"},"steps":[{"step":"login","username":"admin","password":"password"},{"step":"installPlugin","pluginZipFile":{"resource":"url","url":"https://raw.githubusercontent.com/gambitph/Stackable/artifacts/pr3083-stackable-3083-merge.zip"}}]}