Handle situation without contest

Author: vmcj

webapp/src/Service/DOMJudgeService.php

@@ -896,6 +896,9 @@ public function checkIfSamplesZipForContest(Contest $contest): bool
 
         /** @var Contest $contest */
         $contest = $contestQuery->getQuery()->getSingleResult();
+        if (!$contest) {
+            return false;
+        }
 
         if ($contest->getContestProblemsetType()) {
             return true;