[BUGFIX] Fix PHP warning in BackendUtility::getProcessedValue()

BackendUtility::getProcessedValue() fails to initialize a variable which
causes a PHP warning on array access.

Fix this by initializing the variable to array properly.

Resolves: #59994
Releases: 6.3, 6.2
Change-Id: I26711b512a9a7fad40000de6f2de6a2b4a81ac75
Reviewed-on: https://review.typo3.org/31269
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters