[TASK] Drop pointless checks in GridDataService

This change fixes most of the remaining PHPStan warnings
in this class.

Resolves: #97409
Releases: main
Change-Id: I25b6bfbe333e99d3df2df17990649df814ee2afa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74338


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>