[BUGFIX] Evaluate invertStateDisplay in flexforms

The option "invertStateDisplay" for checkboxes was not working with
flexforms, because the received true/false values from a flexform are
of type "string" and not of type "bool".
To correctly evaluate these values, the "invertStateDisplay" options
in the TcaCheckboxItems class are now cast to bool.

Resolves: #90542
Releases: master, 9.5
Change-Id: I52027703a103db858db40095b039837c43ebe5d0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63506


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Susanne Moog <look@susi.dev>