[BUGFIX] htmlArea RTE: Target of localized string may be empty

Solution: Do not allow an empty localized string to overrule the
default string.

Change-Id: Ib20b911560adc022b723985ff35c582de0cf5fc9
Fixes: #33897
Releases: 4.5, 4.6, 4.7
Reviewed-on: http://review.typo3.org/9014
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland