[BUGFIX] Ensure "0" is treated as proper fallback in site language

Due to incomplete handling of "0" and casting, the fallback
is always "0" which is wrong.

The patch fixes the behavior and adds tests to all use-cases
mentioned in the ticket.

Resolves: #88490
Releases: master, 9.5
Change-Id: Ie685b96a94213104729261daa0e3be6e8ef29c35
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60879


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Tobi Kretschmann <tobi@tobishome.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>