Fix ActiveProcess.hasActivity. On active processes, hasActivity tests active_process_uid instead of path:
* hasActivity must handle the case when no document is provided (bug #1142)
* Fix initialisation of active_process_uid property on Message objects.
Add unit test.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@27329 20353a03-c40f-0410-a6d1-a30d3c3de9de