[BUGFIX] TypeConverter cannot resolve disabled FrontendUsers

Change-Id: I63e8be3c210b8db910183dbcb1d0f0edd9ef0de7
Reviewed-on: https://review.typo3.org/31544
Reviewed-by: Sebastian Fischer <typo3@evoweb.de>
Tested-by: Sebastian Fischer <typo3@evoweb.de>