selftests: netfilter: nft_concat_range.sh: send packets to empty set

The selftest doesn't cover this error path:
 scratch = *raw_cpu_ptr(m->scratch);
 if (unlikely(!scratch)) { // here

cover this too.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>