Fix issues with deleted users in phpBB

see:
http://beta.woltlab.com/index.php/Thread/3871-Datenimport-phpBB3x-SQLSTATE-23000-Integrity-constraint-violation-1048-Column-us/
http://beta.woltlab.com/index.php/Thread/3881-Datenimport-phpBB3x-1048-Column-username-cannot-be-null-exportPosts/
http://beta.woltlab.com/index.php/Thread/3883-Datenimport-phpBB3x-1048-Column-username-cannot-be-null-exportConversations/