[BUGFIX] Make sure that only strings get trimExploded

This change fixes places where the code violates the contract.
Also, the change will avoid type errors when the `GeneralUtility`
array functions are switched to native type declarations.

All of these changes fix places where the acceptance test
break with the GU array methods using native type declarations.

Resolves: #101616
Related: #101453
Releases: main, 12.4
Change-Id: I6f86f7a371d50b20a34275ebc771604d407917bf
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80459


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