[BUGFIX] Always return a string for PHP 8.1

The following deprecation occurred in PHP 8.1:
PHP Runtime Deprecation Notice: trim(): Passing null to parameter #1 ($string) of type string is deprecated.

This happens in various places:
* Using stdWrap_field() or in stdWrap() places
* BackendUtility (e.g. new IRRE elements)
* ExtensionManager

Resolves: #96354
Releases: main, 11.5
Change-Id: Ie5c721784b51ed5493c8d9e1d557b08611263f6b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72659


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