According to 13.8 [temp.res] paragraph 8,

If every valid specialization of a variadic template requires an empty template parameter pack, the template is ill-formed, no diagnostic required.

I'm inclined to think that this rule should only apply to code the user wrote. That is, if every valid instantiation of an entity (that was not itself instantiated) requires at least one of the enclosing template argument lists to include an empty template argument pack, then the program is ill-formed (no diagnostic required).