[BUGFIX] Avoid multiple undefined array keys in FileContentParser

When indexing files the FileContentParser collects some metadata e.g. title, description and keywords. For some files these information are not given and the parser breaks in an PHP 8 environment.

Resolves: #100122
Releases: main, 11.5
Change-Id: I54b5512c7e30c9719654591d520943733fbfe920
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78053


Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>