[TASK] Flesh out type declarations in PropertyMapper

Doing so allows us to remove an unnecessary test, as well.

'mixed' type declarations not included to allow backporting.

Used command:

> ./Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #98027
Releases: main
Change-Id: I3c499e557c5d5bc31bf471e577760f4268fa26cc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75295


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>