Fix an NPE in SimpleRequestProcessor when a client request has no invocations.
Issue 5793.
Patch by: bobv
Review by: rchandia
Found by: cory.prowse

Review at http://gwt-code-reviews.appspot.com/1242801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9479 8db76d5a-ed1c-0410-87a9-c151d255dfc7