[BUGFIX] Missing quotes in class_exists parameter

Resolves: #56512
Releases: 6.2

Change-Id: I628dcc6a1e79d7a84306ba06fc8b09a96d920ed7
Reviewed-on: https://review.typo3.org/28020
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen