[BUGFIX] Allow no values in selectCheckbox in DataHandler

With Doctrine DBAL in TYPO3 v8, a default value for
various fields were added to avoid SQL errors (nullable),
setting them to their default value.

However, there is a culprit. When editing e.g.
be_users.file_permissions where you have selectCheckbox field
with all options, it was never possible to unselect all, as
the default value was then used.

Through some workarounds, DataHandler now checks
the modified records on the selectCheckbox type to not
fall back on the default value.

Resolves: #70812
Releases: main, 11.5
Change-Id: I2cd3bf87c31cd4fe5244b26eb3fe2e58e3d8755c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76497


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>