Fix undefined array key warning when using an article list (see #4673)

Description
-----------

-

Commits
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4cbb127 Fix undefined array key warning
1d46c5e Fix the check

Co-authored-by: Leo Feyer <github@contao.org>
Co-authored-by: Martin Auswöger <martin@auswoeger.com>

core-bundle/src/Resources/contao/modules/ModuleArticleList.php

@@ -102,7 +102,7 @@ protected function compile()
 			(
 				'link' => $objArticles->title,
 				'title' => StringUtil::specialchars($objArticles->title),
-				'id' => $cssID[0] ?: 'article-' . $objArticles->id,
+				'id' => ($cssID[0] ?? null) ?: 'article-' . $objArticles->id,
 				'articleId' => $objArticles->id,
 				'href' => $objHelper->getFrontendUrl('/articles/' . ($objArticles->alias ?: $objArticles->id))
 			);