json: Dump flowtable hook spec only if present

json: Dump flowtable hook spec only if present

If there is no priority.expr set, assume hook.num is bogus, too.

While this is fixing JSON output, it's hard to tell what commit this is
actually fixing: Before commit 627c451b23513 ("src: allow variables in
the chain priority specification"), there was no way to detect
flowtables missing hook specs (e.g. when printing flowtable delete
monitor event).

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