missing version:
sed (GNU sed) 4.2.2
case 1: fail
rm -f ./resolv.conf;
touch ./resolv.conf; sed -i 'a nameserver 8.8.8.8' ./resolv.conf ; cat resolv.conf
case 2: ok
rm -f ./resolv.conf;
echo a > ./resolv.conf; sed -i 'a nameserver 8.8.8.8' ./resolv.conf ; cat resolv.conf