[BUGFIX] Invalid fallback for non-localized labels

Whenever a record is not translated, Pootle exports the localization file
with only the "source" element and no "target" element.

Change-Id: Ib4b849abdeff4b045af7a7bcd8550719f814557f
Fixes: #39220
Releases: 6.0, 4.7, 4.6
Reviewed-on: http://review.typo3.org/13020
Reviewed-by: Helmut Hummel
Reviewed-by: Oliver Hader
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers