You certainly don't have to assume it is "empty". JNB is a very busy route for AF. no good or bad answer to your question. What we can say is that there are blocked seats for AF elites. The non-selected seats are released at check-in for all pax. So, for you, it's a gamble : either you purchased a seat for a fee, or you take an average free seat and hope that a better one will be available at OLCI.