Backport "disambiguate overloads for empty index list" to 3.4 branch

Reference issue

Fix #2745 (closed) by backporting !765 (merged) to 3.4 branch.

What does this implement/fix?

It fixes the compilation error documented in #2745 (closed) .

Additional information

OpenCV is a widely used project and package, so it would be great to get this merged before a new release of Eigen is done and is packaged by distributions and package managers.