[BUGFIX] Prevent PHP Warning with TCA selectTree

Make sure there is actually an 'items' array in the processedTCA
before validating the items.

Resolves: #88624
Releases: master, 9.5, 8.7
Change-Id: I1fb6d47ac017ce6950926387387ad19d8445af74
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61122


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>