Fix EditorDriver.setConstraintViolations to not throw NullPointerException when its argument is null. Contributed by tbroyer Fixes issue 6578 Review at https://gwt-code-reviews.appspot.com/1826803/ Review by: cromwellian@google.com git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11261 8db76d5a-ed1c-0410-87a9-c151d255dfc7