[BUGFIX] Avoid PHP warning in TypoScriptFrontendController

TypoScriptFrontendController throws a PHP warning on access to
$BE_USER->user['uid'] which might not be set.
Fix this by simply adding an empty() check.

Resolves: #60126
Releases: 6.3, 6.2
Change-Id: I7005baf73bce55d862b4b6b61697c2aa0a877c7b
Reviewed-on: https://review.typo3.org/31685
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>