[BUGFIX] Avoid undefined array key in RecordListController

Resolves: #96111
Releases: master, 11.5
Change-Id: Id8b381371ea257eb85e2ff59d5aeb1fbb7786db7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72333


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/recordlist/Classes/Controller/RecordListController.php

@@ -309,7 +309,7 @@ class RecordListController
 						<form action="" method="post">';
 
             // Add "clipboard" checkbox:
-            if ($this->modTSconfig['enableClipBoard'] === 'selectable') {
+            if (($this->modTSconfig['enableClipBoard'] ?? '') === 'selectable') {
                 $body .= '<div class="form-check form-switch">' .
                     BackendUtility::getFuncCheck($this->id, 'SET[clipBoard]', ($MOD_SETTINGS['clipBoard'] ?? ''), '', $table ? '&table=' . $table : '', 'id="checkShowClipBoard"') .
                     '<label class="form-check-label" for="checkShowClipBoard">' .