Changes the theme class for SC.EmptyTheme from 'sc-empty' to 'sc-empty-theme'. This fixes the conflict with 'sc-empty' used by SC.ProgressViews that causes all progress views in apps using the empty theme to not have an inner border. This is a potentially conflicting change, but less dangerous than changing the class used by SC.ProgressView, because SC.EmptyTheme doesn't have any styles. Fixes #936