Check if contestProblems collection is empty

Before we would get a `false` as there is no first and fail on:
$iniData = [
	'color' => $contestProblem?->getColor(),
];

as we have a boolean, not a null.

webapp/src/Controller/Jury/ProblemController.php

@@ -247,7 +247,7 @@ public function exportAction(int $problemId): StreamedResponse
             ->getOneOrNullResult();
 
         /** @var ContestProblem|null $contestProblem */
-        $contestProblem = $problem->getContestProblems()->first();
+        $contestProblem = $problem->getContestProblems()->first() ?: null;
 
         // Build up INI data.
         $iniData = [