[BUGFIX] Properly check array access in EXT:indexed_search

Backport parts of #102907 to avoid exceptions because of undefined array
key access.

Resolves: #103997
Releases: 12.4, 11.5
Change-Id: I35e159bd8728fd84a43b58b27dc2a291fd11fe2b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84525


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>