shared/install: fix set-default with empty root (#4118)

https://bugzilla.redhat.com/show_bug.cgi?id=1374371

When root was empty or equal to "/", chroot_symlinks_same was called with
root==NULL, and strjoina returned "", so the code thought both paths are equal
even if they were not. Fix that by always providing a non-null first argument
to strjoina.
(cherry picked from commit ae9efab7)