[BUGFIX] Clearing label through TS doesn't work

Overriding labels of non-default languages using TypoScript
fails if the value is empty.
The problem is that we cannot distinguish between missing translations
and labels cleared by TypoScript.
(In both cases ['target'] === "")

Fixes: #43959
Releases: 6.1, 6.0, 4.7, 4.6
Change-Id: I06697d44a5e4621f8bf45feb5fd3551705af8a2b
Reviewed-on: https://review.typo3.org/19619
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind