Merge pull request #1460 from drwetter/drwetter-patch-1

add also here -z

utils/docker-nginx.tls13-earlydata.start.sh

@@ -6,7 +6,7 @@ ID=$(docker run -d -ti $image)
 
 echo $ID
 
-[[ "$ID" ]] && echo "container couldn't be retrieved" >&2 && exit 1
+[[ -z "$ID" ]] && echo "container couldn't be retrieved" >&2 && exit 1
 
 docker exec -ti $ID nginx -V
 docker exec -ti $ID mkdir /etc/nginx/ssl