[BUGFIX] Use correct fallback for default page TSconfig

The value of a TSconfig entry in the $TSdata array, forwarded
to the ModifyLoadedPageTsConfigEvent, has to be a string.

Therefore the fallback must be a string, too. Otherwise it would
make it unnecessary hard for extension authors to extend the
default TSconfig using the mentioned event as they have to
check for the actual type first.

Resolves: #94718
Releases: master, 10.4
Change-Id: I023a389e79c01bceb427431d15da16d0b2089b0f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70209


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Achim Fritz <af@achimfritz.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>