[BUGFIX] Localization of a constant string is empty

When using a constant string (e.g., "CSS Styled Content") instead of a
localization key (e.g., "LLL:EXT:some_ext/locallang.xml:key"), the constant
string should be returned instead of an empty string.

Change-Id: I409664dc5174d818e875fd03a7e6c6e7fbda9e91
Fixes: #28223
Reviewed-on: http://review.typo3.org/3345
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Dominique Feyer
Tested-by: Dominique Feyer
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers