Fix error when ranking_params is null

modules/contest.js

@@ -249,7 +249,7 @@ app.get('/contest/:id/ranklist', async (req, res) => {
 
         /*** XXX: Clumsy duplication, see ContestRanklist::updatePlayer() ***/
         if (contest.type === 'noi' || contest.type === 'ioi') {
-          let multiplier = contest.ranklist.ranking_params[i] || 1.0;
+          let multiplier = (contest.ranklist.ranking_params || {})[i] || 1.0;
           player.score_details[i].weighted_score = player.score_details[i].score == null ? null : Math.round(player.score_details[i].score * multiplier);
           player.score += player.score_details[i].weighted_score;
         }