fix (image block): allow empty alt tag

[bfintal]

fixes #3376

[github-actions]

🤖 Pull request artifacts

file	commit
pr3404-stackable-3404-merge.zip	42d8eda

[github-actions]

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/pr3404-stackable-3404-merge.zip"}}]}