Code Modernization: Use array_key_first() to read the first key of an array

[Soean]

Reading the first key of an array through array_keys() allocates a full array of every key just to keep one entry and discard the rest. array_key_first() reads the first bucket directly.

Follow-up to 65773, which is scoped to the two nested current( array_keys( $array ) ) call sites and is addressed in #12785. This PR covers a second shape of the same idea, which that ticket does not include: the key array is assigned to a variable first, then read on the next line.

$keys    = array_keys( $wp_registered_sidebars );
$sidebar = reset( $keys );

$sidebar = array_key_first( $wp_registered_sidebars );

13 occurrences across 11 files, in two shapes — reset( $keys ) and $keys[0]. In every case the intermediate variable existed only to carry the key array to the next line and is never read again afterwards.

One change that goes further

In spawn_cron() and _wp_cron() the surrounding check is dropped too:

$keys = array_keys( $crons );
if ( isset( $keys[0] ) && $keys[0] > $gmt_time ) {

if ( array_key_first( $crons ) > $gmt_time ) {

Both functions return early a few lines above when $crons is empty, so isset( $keys[0] ) can never be false there. Happy to keep an explicit null !== $first guard instead if reviewers prefer the defensive form.

Behavior notes

• On an empty array reset() returns false while array_key_first() returns null. None of the touched call sites compares the result with ===, and most sit behind an empty() guard.
• Where the old code used $keys[0], the new code is strictly safer: $keys[0] emitted a notice on an empty array, array_key_first() returns null quietly.

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