A note on this. My emailed tickets show a due date of nothing (NULL) correctly. This is only happening when I reopen a ticket (which is where I added the duedate="NULL")
Ok, I got mine fixed it appears. I changed the reopen function to include duedate=NULL (I had been trying duedate="NULL").
include/class.ticket.php
function reopen(){
global $thisuser;
$sql= 'UPDATE '.TICKET_TABLE.' SET status='.db_input('open').',isanswered=0,duedate=NULL,updated=NOW(),reopened=NOW() WHERE ticket_id='.db_input($th\
is->getId());
return (db_query($sql) && db_affected_rows())?true;
}