chore: avoid using nullsafe operator in php7

Nullsafe operator is supported since PHP8. Avoid using nullsafe operators
in PHP7, as domjudge 8.2 branch needs to support PHP 7.4

Signed-off-by: cubercsl <[email protected]>

Author: cubercsl

webapp/src/Twig/TwigExtension.php

@@ -1111,7 +1111,7 @@ public function problemBadge(ContestProblem $problem): string
     public function problemBadgeForContest(Problem $problem, ?Contest $contest = null): string
     {
         $contest ??= $this->dj->getCurrentContest();
-        $contestProblem = $contest?->getContestProblem($problem);
+        $contestProblem = $contest !== null ? $contest->getContestProblem($problem) : null;
         return $contestProblem === null ? '' : $this->problemBadge($contestProblem);
     }