[BUGFIX] Fix return value in command site:list

Return 0 instead of null.

Resolves: #89936
Releases: master, 9.5
Change-Id: Ic3345975b60fca38537750c9617d68ba9d19cc3e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62621


Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <look@susi.dev>