netfilter: nat: really support inet nat without l3 address

When no l3 address is given, priv->family is set to NFPROTO_INET and
the evaluation function isn't called.

Call it too so l4-only rewrite can work.
Also add a test case for this.

Fixes: a33f387e

 ("netfilter: nft_nat: allow to specify layer 4 protocol NAT only")
Reported-by: Yi Chen <yiche@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>