[BUGFIX] Fix input field with nullable checkbox

The change #99575 introduced a bug in formengine related
to input fields with a "null" checkbox placeholder.

This change fixes the bug by using the original CSS
class in `initializeNullWithPlaceholderCheckboxes()`

Resolves: #99743
Releases: main
Signed-off-by: Torben Hansen <derhansen@gmail.com>
Change-Id: Ie47cd70f062def8ca6c2b62b6e15cfc0cc4ce4de
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77629


Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>