Small fix for elided empty classes
Small fix for elided empty classes

Previously if you had an archive that supported eliding empty classes
(such as binary or portable binary), and specialized that archive there would
be an ambiguity in the enable_ifs for serialization.

guker/cereal,adasworks/cereal,guker/cereal,sschaetz/cereal,arvidsson/cereal,USCiLab/cereal,lyager/cereal,sschaetz/cereal,JosephLaurino/cereal,syxolk/cereal,heamon7/cereal,heamon7/cereal,vcatechnology/cereal,jkhoogland/cereal,syxolk/cereal,RobertoMalatesta/cereal,adasworks/cereal,USCiLab/cereal,lyager/cereal,jonathan-beard/cereal,headupinclouds/cereal,abutcher-gh/cereal,jonathan-beard/cereal,JosephLaurino/cereal,vcatechnology/cereal,fawkes74/cereal,headupinclouds/cereal,abutcher-gh/cereal,RobertoMalatesta/cereal,jkhoogland/cereal,stevehickman/cereal,stevehickman/cereal,fawkes74/cereal,arvidsson/cereal