[BUGFIX] Prevent PHP warning if no config available

If no linkhandler TypoScript configuration is available the
internal configuration array of the ConfigurationManager is
initialized with an empty array to prevent PHP warnings when
the configuration is used in foreach loops.
[BUGFIX] Do not render link label in Frontend

Since the link labels are only needed in the Backend and the
LanguageService and the BackendUtility are not available in the
Frontend the link label generation is now skippen for non Backend
requests.