[BUGFIX] Check for existing category before processing it

The CategoryCollection might be called without an existing category uid
which must not lead to an exception to notices.

Resolves: #99989
Releases: main, 12.4
Change-Id: I6c0bb34d9f5b460f73b8c2933994d94b54d8f01f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84624


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