[BUGFIX] PHP 8 warnings: Undefined array key in EXT:impexp

Some EXT:impexp functional tests threw warnings of the
type "Undefined array key" which did not abort the test
but were written unnoticed to the TYPO3 logs. These
warnings are fixed in this patch.

Resolves: #94972
Releases: master
Change-Id: I0e4f3558cd12ef49ae6dc071edd67cd2621dbc2f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70741


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>