[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