[FIX] base: ValidationError for required m2o with 'set null' on_delete

Before this commit, when creating a many2one field from an xml, with an
impossible combination "`required=True` - `on_delete='set null'`", the
column was not created in the database without any notice to the user.

This commit adds a constraint checking that any attempt to create a field
with this configuration would raise a ValidationError. This constraint
replaces the previous onchange method.

closes odoo/odoo#158778

Signed-off-by: Rémy Voet (ryv) <ryv@odoo.com>

odoo/addons/base/models/ir_model.py

@@ -778,14 +778,14 @@ class IrModelFields(models.Model):
                     'message': _("The table %r if used for other, possibly incompatible fields.", self.relation_table),
                 }}
 
-    @api.onchange('required', 'ttype', 'on_delete')
-    def _onchange_required(self):
+    @api.constrains('required', 'ttype', 'on_delete')
+    def _check_on_delete_required_m2o(self):
         for rec in self:
             if rec.ttype == 'many2one' and rec.required and rec.on_delete == 'set null':
-                return {'warning': {'title': _("Warning"), 'message': _(
+                raise ValidationError(_(
                     "The m2o field %s is required but declares its ondelete policy "
                     "as being 'set null'. Only 'restrict' and 'cascade' make sense.", rec.name,
-                )}}
+                ))
 
     def _get(self, model_name, name):
         """ Return the (sudoed) `ir.model.fields` record with the given model and name.