Fix sub_url=None problem on list_post_links fails
Fix sub_url=None problem on list_post_links fails

When sub_url=None, `url = root_url + sub_url' is throwing exception.

approach0/search-engine,yzhan018/the-day-after-tomorrow,t-k-/the-day-after-tomorrow,approach0/search-engine,yzhan018/the-day-after-tomorrow,t-k-/the-day-after-tomorrow,yzhan018/the-day-after-tomorrow,yzhan018/the-day-after-tomorrow,t-k-/the-day-after-tomorrow,approach0/search-engine,t-k-/the-day-after-tomorrow,yzhan018/the-day-after-tomorrow,t-k-/the-day-after-tomorrow,t-k-/the-day-after-tomorrow,yzhan018/the-day-after-tomorrow,t-k-/the-day-after-tomorrow,yzhan018/the-day-after-tomorrow,approach0/search-engine