json: prevent null deref if chain->policy is not set

json: prevent null deref if chain->policy is not set

The two commits mentioned below resolved null dererence crashes when the
policy resp. priority keyword was missing in the chain/flowtable
specification.

Same issue exists in the json output path, so apply similar fix there
and extend the existing test cases.

Fixes: 5b37479b42b3 ("nftables: don't crash in 'list ruleset' if policy is not set")
Fixes: b40bebbcee36 ("rule: do not crash if to-be-printed flowtable lacks priority")
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>