[BUGFIX] indexed_search: PHP 8 array key exists check

Another PHP 8 sanitation checking for array
key existance.

Resolves: #98040
Releases: main, 11.5
Change-Id: I3f76f619fa9d58b1ce8c6b63314b617be10f8ac5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75315


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>