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.

git-svn-id: https://develop.svn.wordpress.org/trunk@59453 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/compat.php

@@ -415,6 +415,10 @@ function is_countable( $value ) {
 	 *                         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;
 		}

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

@@ -1649,6 +1649,8 @@ protected function handle_terms( $post_id, $request ) {
 				return $result;
 			}
 		}
+
+		return null;
 	}
 
 	/**