Add more error checking to WP_List_Util::pluck

[afragen]

$value is checked to see if it is an object, but the else doesn't check to see if $value is an array.

I've found instances where null is passed to WP_List_Util::pluck() resulting in an error. Some additional error checking in the else should solve this.

Add a check to the else as elseif to ensure $field is an array when it is treated as such.

Trac ticket: https://core.trac.wordpress.org/ticket/56650

@costdev I think I've resolved the above issues.

[audrasjb]

Self assigning for final review.

[audrasjb]

@afragen I added two questions in the PR :)
Otherwise, it looks good to me.

[afragen]

@audrasjb I’m not seeing your questions.

[audrasjb]

@afragen sorry I forgot to submit the review :D

[audrasjb]

Thanks for the follow-up commit ;)
Moving this to milestone 6.2.

[mukesh27]

This ticket was discussed during the recent bug scrub. It looks like it's unlikely that work will be done on this during the 6.2 cycle.

@audrasjb PR ready for re-review. Is this still possible to land in 6.2, or should it be moved to Future Release for now?

[SergeyBiryukov]

In 55423:

General: Add more error checking to WP_List_Util::pluck().

Values for the input array in WP_List_Util::pluck() or wp_list_pluck() must be either objects or arrays.

This commit adds a check to ensure that the value retrieved in the loop is an array before treating it as such, and throws a _doing_it_wrong() notice if it is neither an object nor an array.

Follow-up to [14108], [15686], [18602], [28900], [38928].

Props afragen, costdev, audrasjb.
Fixes #56650.

Thanks for the PR! Merged in r55423.

Note: As wp_list_pluck() is just a wrapper and the actual source code change was in WP_List_Util::pluck(), I have moved the tests to the class where all the other tests for WP_List_Util are located.