[FIX] marketing_campaign: server actions always return None or False, and the return value of method _process_wi_action should return True whatever what the server action returns

If the server actions returned None instead of False, the next activity was not triggered, while it should

addons/marketing_campaign/marketing_campaign.py

@@ -494,11 +494,9 @@ class marketing_campaign_activity(osv.osv):
                               active_ids=[workitem.res_id],
                               active_model=workitem.object_id.model,
                               workitem=workitem)
-        res = server_obj.run(cr, uid, [activity.server_action_id.id],
+        server_obj.run(cr, uid, [activity.server_action_id.id],
                              context=action_context)
-        # server action return False if the action is performed
-        # except client_action, other and python code
-        return res == False and True or res
+        return True
 
     def process(self, cr, uid, act_id, wi_id, context=None):
         activity = self.browse(cr, uid, act_id, context=context)