Coding Standards: Explicitly return `null` instead of coercing `void`.

This addresses two instances where a function that is documented as returning `{someType}|null` doesn't explicitly return `null`.

Affected functions:
* `array_key_first()`
* `WP_REST_Posts_Controller::handle_terms()`

Follow-up to [38832], [52038].

Props justlevine.
See #52217.
Built from https://develop.svn.wordpress.org/trunk@59453


git-svn-id: http://core.svn.wordpress.org/trunk@58839 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/compat.php

@@ -415,6 +415,10 @@ if ( ! function_exists( 'array_key_first' ) ) {
 	 *                         is not empty; `null` otherwise.
 	 */
 	function array_key_first( array $array ) { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.arrayFound
+		if ( empty( $array ) ) {
+			return null;
+		}
+
 		foreach ( $array as $key => $value ) {
 			return $key;
 		}

wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -1649,6 +1649,8 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 				return $result;
 			}
 		}
+
+		return null;
 	}
 
 	/**

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.8-alpha-59452';
+$wp_version = '6.8-alpha-59453';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.