Field 'pf_phpbb_occupation' doesn't have a default value

The field can be different (from set of pf_phpbb_*) depending on the source data for migration. This bug happens due to using static variable in inherited class method and changes in PHP 8.1.0 (https://www.php.net/manual/en/language.variables.scope.php)

As of PHP 8.1.0, when a method using static variables is inherited (but not overridden), the inherited method will now share static variables with the parent method

Fix: https://github.com/phpbb/phpbb/pull/6491