Fix viewing polls with no voters_count key

https://estrogen.network/@froggo/9igjsqqhxhsgtu4q

models.cpp

@@ -86,7 +86,7 @@ void from_json(const json& j, Poll& poll) {
         poll.expires_at = -1;
     }
     j.at("expired").get_to(poll.expired);
-    if (!j.at("voters_count").is_null()) {
+    if (j.contains("voters_count") && !j["voters_count"].is_null()) {
         j.at("voters_count").get_to(poll.voters_count);
     } else {
         poll.voters_count = -1;