Prevent non-null null types from appearing.

Contradiction in terms, and makes the compiler blow up.

http://gwt-code-reviews.appspot.com/1028801/show

Review by: conroy@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9121 8db76d5a-ed1c-0410-87a9-c151d255dfc7