[BUGFIX] Illegal string offset 'uid' in TypoScriptFrontendController

Logoff sets user property to an empty string instead of NULL. This
leads to a PHP Warning in PHP 5.4

Change-Id: Ib0e1a31dbf2fd3fcbcf1e7afd972b53d60c3ab5d
Resolves: #59364
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/30588
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters