Use name as checkbox "widget label" if label is empty

This sets the default for label to name, just like everywhere else.

Otherwise, there is no "widget label" for checkboxes without a custom label (and no label at all if `widget_checkbox_label` is set to `widget`)
Merge pull request #473 from lemoinem/fix/checkbox-widget-label-default

No "widget label" on checkbox if there is no custom label