What would be the easiest way to filter for those files that have no attributes of any kind. Using a variable filter, it looks like I can achieve my goal if I name a specific attribute set, but I'm looking for a way to find those files that are empty with respect to attributes. Thanks.
There is no ready-made filter that processes all Attributes in all Attribute Sets and to find files without any Attribute.
You would have to setup a variable which combines all Attributes from all your Attribute sets and use it in a data-driven category. All files without any Attribute value should then end up in "Other". Not sure how fast this would be, but it should work.