[BUGFIX] Check for sys_category in BE module

Resolves: #63932
Change-Id: I5df4718aabf04128a2b417c094bdcabbaaa5e2de
Reviewed-on: http://review.typo3.org/35549
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>