As per the PHP class basics, The class name can be any valid label, provided it is not a PHP reserved word.
Reserved words are including the lists of keywords, predefined classes and constants, and the list of other reserved words.
'null' is a part of the Predefined constants list, so a class shoudn't be named with this word.
https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/phpbb/cache/driver/null.php#L19

class null extends \phpbb\cache\driver\base