Fixes issue 3560, http://code.google.com/p/google-web-toolkit/issues/detail?id=3560.  Don't attempt to load an image icon if the name is null and prepending package is not requested.  This happens when an SCL returns null as its icon path.

Review by: jat



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5408 8db76d5a-ed1c-0410-87a9-c151d255dfc7