[BUGFIX] Fix PHP warning trigged in getAuthInfoArray()

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

Resolves: #50913
Related: #42921
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: I42ddf8fed715c5f8ce060e6ca5826ef3ed8f223e
Reviewed-on: https://review.typo3.org/24004
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn