Fix fatal error when "admin_post_thumbnail_html" filter is applied with null "$thumbnail_id" param

[kmgalanakis]

Description of the Change

The admin_post_thumbnail_html filter is allowing the $thumbnail_id parameter to be null or undefined but the when a callback for that hook is registered in the plugin, the $thumbnail_id parameter is set to always be defined, thus filter application with just two arguments are throwing fatal errors when the plugin is activated.

Closes #195

How to test the Change

• In the theme's functions.php file add the following command

  add_action( 'init', 'testing_safe_svg' );
  function testing_safe_svg() {
  $content = apply_filters( 'admin_post_thumbnail_html', '', 1 );
  }

Changelog Entry

Fixed - Fatal error when applying the "admin_post_thumbnail_html" with just two arguments.

Credits

Props @kmgalanakis

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [x] All new and existing tests pass.