resolve: include current DNS server in JSON again (#40396)

Fixes a regression caused by c6b6ac63ea.

src/resolve/resolved-manager.c

@@ -2071,6 +2071,12 @@ static int dns_configuration_json_append(
 
         assert(configuration);
 
+        if (current_dns_server) {
+                r = dns_server_dump_configuration_to_json(current_dns_server, &current_dns_server_json);
+                if (r < 0)
+                        return r;
+        }
+
         SET_FOREACH(scope, dns_scopes) {
                 _cleanup_(sd_json_variant_unrefp) sd_json_variant *v = NULL;
 

test/units/TEST-75-RESOLVED.sh

@@ -959,6 +959,9 @@ testcase_10_resolvectl_json() {
 
     assert_eq "$(resolvectl --json=short nta dns0 | jq -rc '.[0].negativeTrustAnchors | .[0]')" 'bar'
     assert_eq "$(jq -rc '.[0].negativeTrustAnchors | .[0]' "$status_json")" 'bar'
+
+    # Test that currentServer is non-empty.
+    jq -rce '.[0].currentServer' "$status_json"
 }
 
 # Test serve stale feature and NFTSet= if nftables is installed