[BUGFIX] Set changed state of FormEngine when null placeholder fields are changed

When a null placeholder checkbox is changed, the linked form field is
now marked as "changed", which triggers the confirmation when leaving
the form while being unsaved.

Resolves: #91351
Releases: master, 9.5
Change-Id: I1b3ac08223a4a4c588a980abe70f22ff9814b13f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64444


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>