Fix nuxeo-directory-radio-group widget missing value selected at document creation

Steps to reproduce:

1. In Studio, create a project with a custom doctype that has a Vocabulary-mapped schema property. Configure its layouts to use nuxeo-directory-radio-group for any UI regarding this field. (attaching sample project .jar)
2. In Web UI, create a new instance of this doctype. Set the radio button field with a value (see screenshot 1)
3. After creation, observe the View tab of the document and note the Metadata layout on the right (screenshot 2).
4. Following this, click the 'Edit document' button from the Document Actions and observe the Edit layout in the opened dialog (screenshot 3).

Expected result: radio group widgets in Metadata and Edit layouts are populated with the value set for the field at document creation (in attached example, should show "Ultra Rare" selected).

Actual result: radio group widgets in Metadata and Edit layouts are missing the value set for the field at document creation - the user cannot tell what value it was given without opening a JSON export of the document and viewing the schema property directly.