[BUGFIX] Cast to string before using GU:trimExplode in indexed_search

As the keyword field can be null, the value must be cast to a string.

Resolves: #102002
Releases: main, 12.4, 11.5
Change-Id: Ic2e437c489c699833c2d51301bdff715f613ea6e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81171


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