inline_validation.js fix

Sometime inline validation view in plone.app.z3cform return 'None' value.
- https://github.com/plone/plone.app.z3cform/blob/master/plone/app/z3cform/inline_validation.py#L21

Now I check the response before rendering error message but maybe p.app.z3cform.inline_validation.InlineValidationView should always return a valid json response.