If Root isn't specified, paths like (null)var/ will be created. I think pacman should either default to / for Root, or just fail if it isn't set.
The backend (although I think this is a frontend problem :) should do
if ( stat(rootdir, &buf) != 0 || ! S_ISDIR(&buf->st_mode) ) Ooops invalid root directory....
Doesn't the frontend already default to / or are you talking about a case where rootdir='' in pacman.conf?