[BUGFIX] Fix PHP 8 warning in AbstractMenuContentObject

Added Null coalescing operator, when value of
$this->mconf['sectionIndex.']['type'] is used.

Resolves: #99866
Releases: main, 11.5
Change-Id: I0b6969b353bf7a55b640f71ced64c06c7539cf27
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77840


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