[BUGFIX] Fix counting of doktypes in DatabaseIntegrityCheck

The previous logic for checking whether a value for a specific
doktype already exists falsely assumed the value should be an
array. This is now fixed by using the null coalescing assignment,
which is available since PHP 7.4.

Resolves: #101196
Related: #94498
Releases: main, 12.4, 11.5
Change-Id: Iab1dba7987d336b08bfb4f398b16666d591daada
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79536


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>