netfilter: nf_tables: Tolerate chains with no remaining hooks

Do not drop a netdev-family chain if the last interface it is registered
for vanishes. Users dumping and storing the ruleset upon shutdown to
restore it upon next boot may otherwise lose the chain and all contained
rules. They will still lose the list of devices, a later patch will fix
that. For now, this aligns the event handler's behaviour with that for
flowtables.
The controversal situation at netns exit should be no problem here:
event handler will unregister the hooks, core nftables cleanup code will
drop the chain itself.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>