[TASK] Simplify DataMapper::getPlainValue()

Use early returns to remove if..else cascades which
are hard to read.

Releases: main
Resolves: #101406
Change-Id: I4e4e559ca469d941dbe7a5c4facad265ab00c3f7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80108


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan B�rk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan B�rk <stefan@buerk.tech>