isisd: fix null display is show isis database detail json

{"mtRouterInfo": "none"} JSON is incorrect.

Change to {"mtRouterInfo": null}

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>

isisd/isis_tlvs.c

@@ -6657,7 +6657,7 @@ static void format_tlvs(struct isis_tlvs *tlvs, struct sbuf *buf, struct json_ob
 
 	if (tlvs->mt_router_info_empty) {
 		if (json)
-			json_object_string_add(json, "mtRouterInfo", "none");
+			json_object_object_add(json, "mtRouterInfo", NULL);
 		else
 			sbuf_push(buf, indent, "MT Router Info: None\n");
 	} else {