[BUGFIX] Fix notice for browse menu with no parent page

Fix a notice if a browse menu is being generated but no parent page
is available.

Resolves: #97608
Releases: main, 11.5
Change-Id: Id0674fddccf62e02febdfa7b609521d0db3114b1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74603


Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: crell <larry@garfieldtech.com>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>