KLS: Don't discard an unresolved empty location during remapping. (#2946)
KLS: Don't discard an unresolved empty location during remapping. (#2946)

This is a follow-up to #2944. In addition to not throwing away empty spans from
the service, also ensure that when those spans are not discarded when they
don't get moved by remapping.
bzz/kythe,kythe/kythe,bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,bzz/kythe,bzz/kythe,bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,kythe/kythe,kythe/kythe,bzz/kythe,bzz/kythe,kythe/kythe,bzz/kythe,kythe/kythe