Loosen the contract on HasValue, and encourage (not require)
use of setValue(null) to clear a widget.

reviewer: ecc
submitter: rjrjr



git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4234 8db76d5a-ed1c-0410-87a9-c151d255dfc7