localrepo.status: ignore empty symlink placeholders
A symlink's target should never be empty in normal use. Solaris and some BSDs
do allow empty symlinks to be created (with varying semantics on dereference),
but a symlink placeholder that started off as empty is either
- going to be empty, in which case ignoring it is fine, since it's unchanged, or
- going to not be empty, in which case this check is irrelevant.