[BUGFIX] Allow sites with only digit numbers as identifier

Fix a missing type cast, where on creating a site with only
a digit number as identifier, both the backend and frontend
would throw a type error.

Resolves: #100083
Releases: main
Change-Id: Ic91d45390449bf7a71779570bd89812fdfd33145
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78025


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>