netfilter: nft_dynset: dump expressions when set definition contains no expressions

If the set definition provides no stateful expressions, then include the
stateful expression in the ruleset listing. Without this fix, the dynset
rule listing shows the stateful expressions provided by the set
definition.

Fixes: 65038428 ("netfilter: nf_tables: allow to specify stateful expression in set definition")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>