Apparently database dump can contain cache codes with no data. Don't crash in that case

okapi.cpp

@@ -151,6 +151,7 @@ Caches Okapi::get_caches(const std::set<std::string>& codes) const {
 		codes_list.clear();
 
 		for (auto& el : j.items()) {
+			if (el.value().is_null()) continue;
 			c.code = el.value()["code"];
 			c.name = el.value()["name"];
 			c.type = el.value()["type"];