Empty fields are not working

Because of

        // Populate $items back in the field values, but avoid replacing missing
        // fields with an empty array (those are not equivalent on update).
        if ($items !== array() || isset($entity->{$field_name}[$langcode])) {
          $entity->{$field_name}[$langcode] = $items;
        }

in the function _field_invoke() there are no empty field values delivered that could then be clicked for editing. But the small change in the attached patch is fixing that.