Posts, Post Types: Fix missing array check in get_post_custom_values().

Posts, Post Types: Fix missing array check in get_post_custom_values().                                          
                                                                                                                   
  `get_post_custom()` can return `false` for an invalid post ID or an empty                                        
  string for a non-existing post. `get_post_custom_values()` accessed the                                          
  result as an array without first verifying it is one, triggering a PHP                                           
  warning on PHP 8+.                                                                                               
   
  Adds an `is_array()` guard consistent with the sibling function                                                  
  `get_post_custom_keys()`, which already handles this case correctly.

Author: saratheonline

src/wp-includes/post.php

@@ -2891,6 +2891,10 @@ function get_post_custom_values( $key = '', $post_id = 0 ) {
 
 	$custom = get_post_custom( $post_id );
 
+	if ( ! is_array( $custom ) ) {
+		return null;
+	}
+
 	return $custom[ $key ] ?? null;
 }