Change emptiness checks to empty() from size() [clang-tidy]

This has two benefits, it's clear what we are checking and
size() isn't always constant time, where empty is (performance!)

zone/aa.cpp

@@ -1141,7 +1141,7 @@ void Client::ActivateAlternateAdvancementAbility(int rank_id, int target_id) {
 	}
 
 	//make sure it is not a passive
-	if(rank->effects.size() > 0) {
+	if(!rank->effects.empty()) {
 		return;
 	}