[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'] === "")

Change-Id: Iac666ecd7fa9af900f9852a777be3127437e0365
Fixes: #43959
Releases: 6.2, 6.1, 6.0, 4.7
Reviewed-on: https://review.typo3.org/20354
Reviewed-by: Stephan Großberndt
Tested-by: Stephan Großberndt
Reviewed-by: Markus Klein
Tested-by: Markus Klein