fix(models): allow nullable language on LeaderboardModel

Change the language relation on LeaderboardModel to be nullable to handle
cases where a leaderboard may not have an associated language. Update the
preferred-language-leaderboard service to filter out leaderboards with
null languageId and safely access the language slug. This prevents runtime
errors and ensures that only valid leaderboards are considered for the
preferred language leaderboard calculation.

Author: rohitpaulk

app/models/leaderboard.ts

@@ -5,7 +5,7 @@ import type LanguageModel from './language';
 
 export default class LeaderboardModel extends Model {
   @belongsTo('contest', { async: false, inverse: 'leaderboard' }) declare contest: ContestModel;
-  @belongsTo('language', { async: false, inverse: 'leaderboard' }) declare language: LanguageModel;
+  @belongsTo('language', { async: false, inverse: 'leaderboard' }) declare language: LanguageModel | null;
 
   @hasMany('leaderboard-entry', { async: false, inverse: 'leaderboard' }) declare entries: LeaderboardEntryModel[];
 

app/services/preferred-language-leaderboard.ts

@@ -77,9 +77,11 @@ export default class PreferredLanguageLeaderboardService extends Service {
 
     this.preferredLanguageSlugs = userLeaderboardEntries
       .filter((entry) => entry.score > 0)
+      // @ts-expect-error: languageId not defined on LeaderboardModel
+      .filter((entry) => entry.leaderboard.languageId !== null)
       .sort((a, b) => b.score - a.score)
-      .map((entry) => entry.leaderboard.language.slug)
-      .slice(0, 3);
+      .slice(0, 3)
+      .map((entry) => entry.leaderboard.language!.slug);
 
     this.localStorage.setItem(PreferredLanguageLeaderboardService.STORAGE_KEY, new StoredData(this.preferredLanguageSlugs, new Date()).toJSON());
   }