Fix 0010918: Undefined $t_null_date when updating due date field

Preconditions:

• User has permission to update the due date
• The due date field is configured to show on the update page

When these preconditions are true, an error is thrown due to an invalid
check for a null date. We should be using the date_is_null function
instead as this is what is used in other parts of Mantis to check for
null due dates.