[BUGFIX] Avoid calling strlen(null) in FrontendUserAuthentication

GeneralUtility::_POST($this->formfield_permanent) returns
null if that field is not set in $_POST. Feeding null
to strlen() emits a E_DEPRECATED PHP 8.1 error. Sanitize
that call.

Change-Id: Ib2712ee04807f2853f6dc4b66c76293ce6e161e8
Resolves: #95765
Releases: master
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71967


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