Don't ever set the "base" variable to null.  It is supposed to be the empty string
if it hasn't been set yet.

Review at http://gwt-code-reviews.appspot.com/219801

Review by: jgw@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7736 8db76d5a-ed1c-0410-87a9-c151d255dfc7