[BUGFIX] Ignore prefixParentPageSlug for non-page record in SlugHelper

TCA slug field generator option 'prefixParentPageSlug' has been added
to be used for page records only. Slug generation for non-page records
with pid=0 has been fixed with #94655 which introduced a endless loop
if 'prefixParentPageSlug' is used as slug field generator option for
record tables.

This patch only process 'prefixParentPageSlug' for page records, thus
ignoring it for non-page records and avoiding the endlees loop and as
such prevent segmentation faults or memory_limit fatal errors.

Resolves: #95485
Related: #94655
Releases: master, 11.5, 10.4
Change-Id: I20e1ac68bad6ad9447b03debbedb646495045ac3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72168


Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>