[BUGFIX] Make null-value checkboxes clickable

The checkbox "t3-form-field-eval-null-checkbox" lost its former CSS class
"checkbox" and thus its attribute "position: relative". This happened
during the recent change to streamline checkboxes in the TYPO3 Backend.

This change adds the missing attribute, so that the checkbox is no longer
obscured by the "t3-form-field-disable" overlay, which prevented all
mouse interactions.

Resolves: #94817
Related: #93310
Releases: master
Change-Id: I1fba185f58cd927eb186e39feab5c6d9c783315a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70451


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Benni Mack <benni@typo3.org>