`WP_Image_Editor_GD` and `WP_Image_Editor_Imagick` should specify type in a doc block for the `$image` property, instead of inferring `bool` and `null`.

See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30180 602fd350-edb4-49c9-b593-d223f7449a82