[BUGFIX] Avoid passing null as parameter to trim()

To make units test notice free in #85092 a wrong
fallback for array access has been used.

Fallback to null and passing to trim() through
null coalescing operator has been the wrong choice,
as it triggers a deprecation message in PHP 8.1.

This patch change the fallback to an empty string
instead to avoid this deprecation message.

Resolves: #95747
Related: #85092
Releases: master, 10.4
Change-Id: Ica8522a18d5980affe87958c6b662ce4c1b64578
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71950


Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>