Fix handling of null passed into varargs.

Fixes a bug where passing null into a varargs method in web mode would create a 1-length array containing null.  The correct behavior is to pass null in for the array.

Found by: rchandia

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9574 8db76d5a-ed1c-0410-87a9-c151d255dfc7