[BUGFIX] Apply class attribute of viewhelper correctly

Change-Id: Ia93f3724809e18b01c6b94632a13196c65f216c0
Resolves: #57233
Reviewed-on: https://review.typo3.org/28702
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
(cherry picked from commit 5e22ec527aa518b73ef06f6045073e92954bd743)
Reviewed-on: https://review.typo3.org/28720