[BUGFIX] Initialise parameter in AbstractElementBrowser correctly

This avoids the deprecation notice in PHP 8.1:
Passing null to parameter #2 ($string) of type string is deprecated in /var/www/html/public/typo3/sysext/recordlist/Classes/Browser/DatabaseBrowser.php line 179

Resolves: #96422
Releases: main, 11.5
Change-Id: I9c811bc0d2d4523a78fc6c72ee02be96efbe1bc3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72764


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>