[FIX] project: avoid to remove the default partner_id

The onchange of the project field
removed the default partner loaded of the task
if there was no project set on the task, or if the
project was not associated to a specific partner.

There is no obvious reasons to do so,
and the revision 3fe06da634
introducing the regression doesn't explain why
it has been done. At first sight, there is no reason.

opw-657876

addons/project/project.py

@@ -388,7 +388,7 @@ class task(osv.osv):
             project = self.pool.get('project.project').browse(cr, uid, project_id, context=context)
             if project and project.partner_id:
                 return {'value': {'partner_id': project.partner_id.id}}
-        return {'value': {'partner_id': False}}
+        return {}
 
     def onchange_user_id(self, cr, uid, ids, user_id, context=None):
         vals = {}