[BUGFIX] Type cast PageView return value to string

In order to prevent a TypeError, the rendered
view is now casted to string. This is done,
since the result might be `NULL`, e.g. in
case of using an empty template file.

Resolves: #103537
Releases: main
Change-Id: I59272fec4b9514f38bb97bc4aff17e86d67ec1ae
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83651


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>