certificates: check for contents before rm of certs

Check that there are any contents within /etc/ssl/certs prior
to the `rm -r` call. This removes a spurious, known warning that
the directory is empty, and there is nothing to remove.

Relates to https://gitlab.com/gitlab-org/charts/gitlab/issues/1534

alpine-certificates/scripts/bundle-certificates

 #!/bin/sh
 
 # purge any existing state
-rm -r /etc/ssl/certs/*
+if [ $(ls -1 /etc/ssl/certs/ | wc -l) -gt 0 ]; then
+  rm -r /etc/ssl/certs/*
+fi
 
 # Update the CA certificates store in /etc/ssl/certs
 # - generated hashes