optimize: fix incorrect expansion into concatenation with verdict map

optimize: fix incorrect expansion into concatenation with verdict map

 # nft -c -o -f ruleset.nft
 Merging:
 ruleset.nft:3:3-53:          meta pkttype broadcast udp dport { 67, 547 } accept
 ruleset.nft:4:17-58:         meta pkttype multicast udp dport 1900 drop
 into:
        meta pkttype . udp dport vmap { broadcast . { 67, 547 } : accept, multicast . 1900 : drop }
 ruleset.nft:3:38-39: Error: invalid data type, expected concatenation of (packet type, internet network service)
                meta pkttype broadcast udp dport { 67, 547 } accept
                                                   ^^

Similar to 187c6d01d357 ("optimize: expand implicit set element when
merging into concatenation") but for verdict maps.

Reported-by: Simon G. Trajkovski <neur0armitage@proton.me>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>