[BUGFIX] Avoid coalescence to null for string expecting functions

Various functions expect string arguments and emit
E_DEPRECATED with PHP 8.1 in non strict_types=1
calls. The patch adjusts some null coalescence
fallbacks from null to string to satisfy this.

Resolves: #95787
Releases: master
Change-Id: Ie1b28bc285c33af3509d18eaeb7d0b6013284434
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71983


Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>