can-value shows "undefined" if bound to undefined value

When using the super-cool `can-value`-attribute in mustache, I recently ran into the situation of a database-model, that would actually only contain fields that contained data, i.e. that not set the rest of the fields to some default or undefined.

While (probably) saving some conneciton-overhead (think json with 500 empty + 2 non-empty fields vs. just 2 fields), this happens to make `can-value` show `undefined` in the view - something you would never want to show to the user :-)

This pull-request will fix only the view-binding, i.e. the value still is `undefined` (which is totally intended!).