[TASK] Simplify DataMapFactory::setRelations()

In order to have this method more readable, elseif statements
are broken apart and replaced with early returns.

Releases: main
Resolves: #100193
Change-Id: Iea89286280229f7e872c02d75893c4246c6865c0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78144


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Markus Klein <markus.klein@typo3.org>