Frdige: hide magnet bag when empty.

src/server/gwt/src/se/chalmers/cs/gf/gwt/public/Fridge.css

@@ -34,6 +34,10 @@ body {
   border: 3px solid #dddddd;
 }
 
+.my-FridgeBagPanel-empty {
+  visibility: hidden;
+}
+
 .my-PrefixPanel {
   margin-bottom: 1em;
 }