In the converter of Many2one relations, the model name must be a keyword

argument, optional and not compulsory, as in Odoo 11. Moreover, when the Many2one is empty and a value has to be returned, the converter tries to return the class of the Many2one model. The converter must be able to automatically identify the model name from the Many2one definition, without requiring the model name to be given in arguments.

--HG-- branch : 13.0

Development associated to issue 524.