[BUGFIX] Make check for last key work for menu items with key = 0

Additionally switch $nextActive to boolean

Resolves: #100995
Releases: main, 12.4
Change-Id: I0e41835f42753595480afb16182c446991157ec2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79258


Tested-by: core-ci <typo3@b13.com>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>