Changes output of `wp_get_post_content_block_attributes` to return null if Post Content block doesn’t exist or empty array if it has no attributes.
Props flixos90, mukesh27.
Fixes#59358.
git-svn-id: https://develop.svn.wordpress.org/trunk@56629 602fd350-edb4-49c9-b593-d223f7449a82