[BUGFIX] Followup: [CLEANUP] BackendUtility

With the cleanup for BackendUtility the possible return value of
getProcessedValue() changed. As multiple functions depend on the
fact that the result could be NULL this breaks existing functionality.

Make default return value NULL instead of an empty string.

Resolves: #62414
Releases: master
Change-Id: I01292ec514d5440fac2e4ddaa851607bcf2cb86e
Reviewed-on: http://review.typo3.org/34416
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>