resolve/mdns: do not append packets without records

Previously, if we have only max_rr cached entries, then an empty packet
is appended.

Prompted by #29168.

(cherry picked from commit 6cfa4fc)
(cherry picked from commit 05e31c7)

Author: yuwata

src/resolve/resolved-dns-cache.c

@@ -1271,6 +1271,19 @@ int dns_cache_export_shared_to_packet(DnsCache *cache, DnsPacket *p, usec_t ts,
                         if (usec_sub_unsigned(j->until, ts) < j->rr->ttl * USEC_PER_SEC / 2)
                                 continue;
 
+                        if (max_rr > 0 && ancount >= max_rr) {
+                                DNS_PACKET_HEADER(p)->ancount = htobe16(ancount);
+                                ancount = 0;
+
+                                r = dns_packet_new_query(&p->more, p->protocol, 0, true);
+                                if (r < 0)
+                                        return r;
+
+                                p = p->more;
+
+                                max_rr = UINT_MAX;
+                        }
+
                         r = dns_packet_append_rr(p, j->rr, 0, NULL, NULL);
                         if (r == -EMSGSIZE) {
                                 if (max_rr == 0)
@@ -1296,18 +1309,6 @@ int dns_cache_export_shared_to_packet(DnsCache *cache, DnsPacket *p, usec_t ts,
                                 return r;
 
                         ancount++;
-                        if (max_rr > 0 && ancount >= max_rr) {
-                                DNS_PACKET_HEADER(p)->ancount = htobe16(ancount);
-                                ancount = 0;
-
-                                r = dns_packet_new_query(&p->more, p->protocol, 0, true);
-                                if (r < 0)
-                                        return r;
-
-                                p = p->more;
-
-                                max_rr = UINT_MAX;
-                        }
                 }
 
 finalize: