Formatting: Fix wp_list_pluck() dropping index keys equal to zero

[haritpanchal]

Summary

WP_List_Util::pluck() used if ( ! $index_key ) to decide whether an index key was provided. Since 0 is falsy in PHP, passing 0 as $index_key was treated identically to not passing one at all, silently dropping the requested keys. array_column() — which this method's docblock says it mirrors — doesn't have this problem.

Reproduction (from the ticket)

var_dump( wp_list_pluck( [ [ 'key1', 'val1' ], [ 'key2', 'val2' ] ], 1, 0 ) );
// Before: array(2) { [0]=> "val1" [1]=> "val2" }   -- keys lost
// After:  array(2) { ["key1"]=> "val1" ["key2"]=> "val2" }

Changes

- wp-includes/class-wp-list-util.php: if ( ! $index_key ) → if ( null === $index_key ).
- Added two regression tests to tests/phpunit/tests/functions/wpListPluck.php covering array and object input, both with a 0 index key.

Testing

- Confirmed both new tests fail on unpatched trunk (reproducing the reported bug) and pass with the fix.
- Ran the full Tests_Functions_wpListPluck / WP_List_Util suite (87 tests) — all pass.
- Ran the taxonomy test group (903 tests, a heavy consumer of wp_list_pluck()) as a broader regression check — all pass.
- Verified no existing core call site passes 0 or '' as $index_key (grepped all 65 usages), so this fixes the bug with no behavior change to existing callers.
- phpcs clean on both changed files.

Trac ticket: https://core.trac.wordpress.org/ticket/57136

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Identifying the root cause via source review, implementing the fix, writing and verifying the regression tests (confirmed red→green), and running the broader test sweep. Reviewed and submitted by me.

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