screenShield: Remove erroneous top/bottom padding that may appear

If there are either no resident or persistent notifications, we'll
add some spacing for those boxes that may contain nothing. Make them
invisible to remove the spacing for those elements. It's possible
that we may want to be smarter about this in StBoxLayout to remove
spacing for zero-sized actors, but today is not the day.

https://bugzilla.gnome.org/show_bug.cgi?id=685919