[TASK] Fix undefined array key warning in free mode

$rowsByLang[0] is not set, if editing a content element without a
language source. A check for existence is added to prevent warnings in
PHP 8.

Resolves: #95059
Releases: master
Change-Id: I200175d4127d8925973022c6b3eed86ce1f575b9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70846


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