[BUGFIX] Avoid calling non-existing method on `NullSite`

`\TYPO3\CMS\Core\Site\Entity\Site::isTypoScriptRoot()`
has been added with #103437 without adding it to the
`\TYPO3\CMS\Core\Site\Entity\NullSite` class and the
`\TYPO3\CMS\Core\Site\Entity\SiteInterface` and used
in `PageInformationFactory::setSysTemplateRows()`.

Calling this method requires that the concrete Site
implementation needs to be guaranteed, which is not
done with introduced usage with #103439 in method
`PageInformationFactory::setSysTemplateRows()`.

This change adds now a interface check to call the
method only on concrete `Site` entity objects.

Resolves: #103554
Related: #103437
Related: #103439
Releases: main
Change-Id: I8f1201d7c3e7990a3d7464de2d6d921c7b5de605
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83693


Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>