[DX] Melodiia FormErrorResponse should support non submitted form

[Nek-]

Currently something like this is required:

        if ($form->isSubmitted() && !$form->isValid()) {
            return new FormErrorResponse($form);
        }

        if (!$form->isSubmitted()) {
            return new FormNotSubmittedResponse();
        }

The FormErrorResponse should return something also for non-submitted form.