[sobjects/notification] fix broken email notifications
In https://hg.logilab.org/review/cubicweb/rev/8caa109dfe945592a34904bb56971cc57d1c6084 EntityView.w become a method using EntityView._w
In notifications we reset "self.w" for every notifications since we handle multiple streams.
We should set "self._w" to None instead, otherwise subsequent calls to "self.w()" will fail with TypeError: 'NoneType' object is not callable.
