Avoid signaling errors from ls-lisp--insert-directory on macOS

* lisp/ls-lisp.el (ls-lisp-sanitize): Don't assume the
directory entries for ".." and "." will either both be present
or both absent.  (Bug#37236)
1 job for master in 57 minutes and 25 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #3161
test-all

00:57:25