Yoast/wordpress-seo — pull request #594 by mpatek (2014-01-21 16:33:21+00:00)
Title: Empty value for unchecked checkbox.

Note the logic for displaying checkbox fields [here](https://github.com/Yoast/wordpress-seo/blob/master/admin/class-metabox.php#L566)

In order for checkbox metabox fields to work, the unchecked state should be saved as a value that evaluates to `false`.