[BUGFIX] Fix undefined array key "pages" in FileContentParser

Fix undefined array key "pages" in FileContentParser by adding
a null coalescing operator.

Resolves: #101450
Releases: main, 12.4, 11.5
Change-Id: Idc3bba5d5ab5516689925fadec86e6fac5978e73
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80544


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