Document name should not be mandatory on REST API

Nuxeo Web UI forces users to fill the title property whereas this is not a mandatory field in the schema definition. This leads to inconsistency while building custom logics with Studio (see related ticket). Moreover, the title property is never set as mandatory on custom document type so the user doesn't exactly know what's happening when the document is not created without title.

In Designer:

1. Create a custom doctype
2. Configure creation layout for it
3. Set title as an optional field

Save and deploy
=> It is impossible to create the document due to client side validation, although the schema does not make dc:title a required field and product can handle this; was a potential use case in JSF

=> Prevents typical use case where title is set automatically through business logic, for instance through an event handler