[BUGFIX] Undefined array key in ElementInformation

A click on inline fal ElementInformation button leads to
Undefined array key "returnUrl".
Fixed by adding empty string as fallback and add an
acceptance test to make sure it will not appear again.

Resolves: #94400
Releases: master
Change-Id: I7e44b026f81b0d484e3b8a1af5522fece228d5d4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69544


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>