Only get freezedata on existing contests

This is a annoying case, we only check if the contests exists after we
know if the user has access. This prevents us from disclosing data but
does require this null check.

Author: vmcj

webapp/src/Controller/API/ContestController.php

@@ -364,7 +364,7 @@ public function problemsetAction(Request $request, string $cid): Response
 
         $hasAccess = $this->dj->checkrole('jury') ||
             $this->dj->checkrole('api_reader') ||
-            $contest->getFreezeData()->started();
+            $contest?->getFreezeData()->started();
 
         if (!$hasAccess) {
             throw new AccessDeniedHttpException();