PHP82 deprecated error in MarkdownExtra doExtraAttributes

PHP Deprecated:  preg_match_all(): Passing null to parameter 2 ($subject) of type string is deprecated in /var/www/dprp/vendor/michelf/php-markdown/Michelf/MarkdownExtra.php on line 232

Line 227 checks for empty $attr && no default ID value && empty classes, but line 232 still tries to match on $attr, which could be null.

Using `$attr ?? ""` on line 232 probably solves that.