Fix the usage of the 'manage_posts_custom_column' hook

[justinmaurerdotdev]

Fixes #1068.

Description

Per the #1068 issue, there are some use cases where the 'manage_posts_custom_column' hook is used outside of The Loop. In current versions of the plugin, this scenario produces an empty column. This PR refactors the callback for that hook to use the passed $post_id instead of using the global $post variable.

Testing

You can test that nothing has broken by viewing the Posts table and checking that the Authors column displays the same as before. I'm not 100% sure of a reliable test for the edge case, besides installing a plugin like Admin Columns and running the export from the Posts table.

[GaryJones]

Thanks for this, @justinmaurerdotdev — it's the right fix for a still-live bug (#1068), where the Authors column reads global $post and blows up when a list table is rendered outside The Loop (Admin Columns exports and similar).

I've taken it over in #1309: your commit is cherry-picked across with authorship preserved and rebased onto current develop, so it keeps the newer data-* byline markup that had been added to the same method since you opened this. I also added a regression test that renders the column from a post ID with no global $post set (it fails without your fix and passes with it).

Closing in favour of #1309. Credit for the fix is yours — thank you.