Require related object for content-filtering on notification

When server-side filtering expressions are defined, a join with object table is attempted to filter on status content. In case of follow*/move notifications there is no related object id however, just the user ap_id, so it will end up being a regexp match against null, which always fails, confusingly removing all non-mention/own post interaction notifications from the api results.

This MR limits filtering only to notifications that can be related to some object with non-null content field.