[BUGFIX] Fix PHP warning trigged in initFEuser

TypoScriptFrontendController::initFEuser() calls
$GLOBALS['TYPO3_DB']->cleanIntList() with a possible NULL argument.
This leads to a PHP warning in GeneralUtility::trimExplode().

Resolves: #42921
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: Id33ee8fe5e4177716af6ce8c9084e098b86c280c
Reviewed-on: https://review.typo3.org/22340
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters