Fixed querying of custom metadata superset attributes.
There was a bug where if you did not specify any system
attrs, it would never be in the list passed to the custome
query function, and thus never getting ot query custom metadata.
Fix: Added the uri to the specific attr list so that it would be
included. the uri is then not included in the final list of dicts,
since it is not in the original attr list/
We then use a list comprehension to delete any empty dicts.