Optional Model Values don't work right

(Bug, Investigate , Priority: Medium, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Editor, Model Types
Summary: Create a Model, with a Text field. Then, in another Thing, create an Optional Property derived from that Model. Fill in the text field. Restart the server -- the value is not correctly persisted.
This would be High Priority, except that Optional Model Properties are simply a bad idea to begin with right now: there is no way in the UI to specify that the value should be None, so they're kind of pointless.