Required attribute not enforced on radio-group with non-persistent binding

The nuxeo-directory-radio-group widget doesn't enforce the required attribute when the value attribute is given a non-persistent binding such as "ind_info_complete", as opposed to "document.properties.CAP_STM-SCH-facture:ind_info_complete".

How to reproduce:

**Configure a radio-group widget in a Create or Edit layout for a document type through Studio Designer with non-persistent binding; attempt to Save the form without selecting a radio-group option in Web UI.