[BUGFIX] Correctly check TSFE->rootLine

The existing check on the data-type of $rootLine has to be adjusted
since the default value is now an array.

Resolves: #89512
Related: #89482
Releases: master, 9.5
Change-Id: Ia1d673ff2fe1f45e615c5bc6c81b574b63ec2e54
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62112


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>