And I believe was addressed back then.
So I could just press <enter> without entering any filter value and get my agenda filtered to show only empty tags.
Unfortunately with release 20160118 or Org I get wrong result with this - I just get all the item without any filtering at all.
So I have to resort back to entering -{.} as filtering criteria to only see items in agenda which have no tags assigned to them.
Not very intuitive, don't you think?