[BUGFIX] Type safe check for $BE_USER in index_ts.php

Resolves: #41593
Releases: 6.2
Change-Id: I3863789ad2489055169f3f008cb4dac0085fde82
Reviewed-on: https://review.typo3.org/27463
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Marcin Sągol
Reviewed-by: Michał Jankiewicz
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters