Cannot un-set the node trail for a node

Once a Node Trail item has been selected on a node, you cannot remove that setting.

To reproduce:

• Create a node and choose an item for the node trail
• Edit the node, and change the node trail to <Primary Links> or another parent menu, then save
• Edit the node again, and you'll see that the node trail is still set to the menu item it was originally set to

I'll be working on this later today or tomorrow and will post a patch then. If someone else beats me to it, even better!