[BUGFIX] Avoid notice in ContentObjectRenderer::listNum

Resolves: #101925
Releases: main, 12.4, 11.5
Change-Id: Icec9f634ca1e1a9c7ad6b3a55afaf197cfa44acc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81045


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>