Another option is embedding some JS code in the Due Date description, to access its content and if empty, set now+14 days ...
Some examples here: http://www.j-tricks.com/1/post/2011/12/one-fixversion-please.html
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events