If no configurable Site exists, show a message to the user
that explains why nothing exists.

Resolves: #104913
Releases: main
Change-Id: I6e1c39a19d79c82a1552b3d35610b89d6f1b7ba1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86021


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: Benjamin Franzke <ben@bnf.dev>