[TASK] Show unassigned sites in site module

The site module displays now also sites with no valid related page anymore.
This can happen if the page is being removed.

Resolves: #86150
Releases: master
Change-Id: If75a33e6c2d189abd42746b52d0b828b7a7b38cb
Reviewed-on: https://review.typo3.org/58192


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Riny van Tiggelen <info@online-gamer.nl>
Reviewed-by: Timo Hund <timo.hund@yahoo.com>
Tested-by: Timo Hund <timo.hund@yahoo.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>