[TASK] Allow sys_category in rootpage

Allow sys_category records to be stored in the rootpage. (pid=0)

Resolves: #47530
Releases: 6.2, 6.1, 6.0
Change-Id: Ie71a13ec9a1be65c57e28fff63f003fc5caa6ac1
Reviewed-on: https://review.typo3.org/20123
Reviewed-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Oliver Hader
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind