jp, plugin merge-requests: used u'' as default for item id in pubsub arguments + fixed some required arguments: u"" is now used as defaut item id instead of node, as it needs to be serialised anyway. This is temporary solution, as empty string can be used for item id (not forbidden by XEP-0060), but for now this is not handled correctly in jp/SàT (see https://bugs.goffi.org/bugs/view/289). Correct solution will probably need to use a special key in extra to mark empty string in id. Added/removed some required values for node or item when it made sense.