Bugfix: Set overview/header date field to null if not set

Some emails I receive seem to be missing a date, resulting in an exception:
exception 'ErrorException' with message 'Undefined property: stdClass::$date' in /var/www/vendor/tedivm/fetch/src/Fetch/Message.php:205
exception 'ErrorException' with message 'Undefined property: stdClass::$date' in /var/www/vendor/tedivm/fetch/src/Fetch/Message.php:283

This fixes the problem by setting the fields date and udate to null