[BUGFIX] Illegal string offset warning in AjaxLoginHandler

After logout $GLOBALS['BE_USER']->user['uid'] is checked.
This is done without isset() which triggers a warning, as ->user is unset,
if the logout was successful.

Resolves: #49327
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: Ia13af7f2a75090050113ff6a5d388b790df0bab1
Reviewed-on: https://review.typo3.org/21570
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters