[TASK] Fix a PHP warning in Clipboard

Check if the key is available.

Resolves: #57300
Releases: 6.2
Change-Id: I99542a8c0516596b31342a7bce5daa58af4a6432
Reviewed-on: https://review.typo3.org/28777
Reviewed-by: Alexander Stehlik
Reviewed-by: Sebastian Fischer
Reviewed-by: Oliver Klee
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer

typo3/sysext/backend/Classes/Clipboard/Clipboard.php

@@ -429,7 +429,7 @@ class Clipboard {
 	 */
 	public function hasElements() {
 		foreach ($this->clipData as $data) {
-			if (is_array($data['el']) && !empty($data['el'])) {
+			if (isset($data['el']) && is_array($data['el']) && !empty($data['el'])) {
 				return TRUE;
 			}
 		}