fix clearing ap cache of local magazines

[e-five256]

right now accepting a moderator request or deleting a moderator for a local magazine results in a 500:

hash(): Argument #2 ($data) must be of type string, null given
in [src/EventSubscriber/ActivityPub/MagazineModeratorAddedRemovedSubscriber.php ](file:///var/www/mbin/src/EventSubscriber/ActivityPub/MagazineModeratorAddedRemovedSubscriber.php#L57)(line 57)
$this->cache->delete('ap_'.hash('sha256', $magazine->apId));