Tests: Fix unit test failure in PHP 8.5 due using null as an array offset.

This adds `is_string()` type checks for the `$post_status` arg passed to both `get_post_status_object()` and `is_post_status_viewable()`. This was triggered by `Tests_Post_IsPostStatusViewable::test_built_in_and_unregistered_status_types()` for the `null` data set. 

Follow-up to [61170].

See #63167, #63061.


git-svn-id: https://develop.svn.wordpress.org/trunk@61171 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/post.php

@@ -1512,7 +1512,7 @@ function register_post_status( $post_status, $args = array() ) {
 function get_post_status_object( $post_status ) {
 	global $wp_post_statuses;
 
-	if ( empty( $wp_post_statuses[ $post_status ] ) ) {
+	if ( ! is_string( $post_status ) || empty( $wp_post_statuses[ $post_status ] ) ) {
 		return null;
 	}
 
@@ -2455,6 +2455,10 @@ function is_post_type_viewable( $post_type ) {
  */
 function is_post_status_viewable( $post_status ) {
 	if ( is_scalar( $post_status ) ) {
+		if ( ! is_string( $post_status ) ) {
+			return false;
+		}
+
 		$post_status = get_post_status_object( $post_status );
 
 		if ( ! $post_status ) {