Weboob connector: use the label if no raw provided + ensure it is not empty...

Instead of having an empty raw label we had the string NotLoaded because we did not check if the raw label was empty before calling unicode() on it (see https://git.weboob.org/weboob/weboob/blob/master/weboob/capabilities/base.py#L216).

We should also fallback on the label field if there is no raw field (happened with ING connector for a moment) for stability.