Coding Standards: Use array_first() to get the first array element.

This replaces `array_values( ... )[0]` and `array_slice( ..., 0, 1 )[0]` constructs with the `array_first()` function added in PHP 8.5 for improved readability.

WordPress core includes a polyfill for `array_first()` on PHP < 8.5 as of WordPress 6.9, so the change works on every supported PHP version without raising the minimum requirement.

Follow-up to [9685], [50995], [60672].

Props Soean, mukesh27.
See #65598.

git-svn-id: https://develop.svn.wordpress.org/trunk@62695 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-walker.php

@@ -234,8 +234,7 @@ public function walk( $elements, $max_depth, ...$args ) {
 		 */
 		if ( empty( $top_level_elements ) ) {
 
-			$first = array_slice( $elements, 0, 1 );
-			$root  = $first[0];
+			$root = array_first( $elements );
 
 			$top_level_elements = array();
 			$children_elements  = array();

src/wp-includes/rest-api/endpoints/class-wp-rest-widget-types-controller.php

@@ -504,7 +504,7 @@ public function encode_form_data( $request ) {
 			isset( $request['form_data'][ "widget-$id" ] ) &&
 			is_array( $request['form_data'][ "widget-$id" ] )
 		) {
-			$new_instance = array_values( $request['form_data'][ "widget-$id" ] )[0];
+			$new_instance = array_first( $request['form_data'][ "widget-$id" ] );
 			$old_instance = $instance;
 
 			$instance = $widget_object->update( $new_instance, $old_instance );