[BUGFIX] Use correct fallback type for GeneralUtility::sanitizeLocalUrl

`GeneralUtility::sanitizeLocalUrl` expects a string. So
the fallback value when calling this method also needs
to be a string, not null. This patch adjusts two of
such calls.

Resolves: #101310
Related: #101305
Releases: main, 12.4, 11.5
Change-Id: I737eea4910e95f653982cbe5b268c782cdf8a502
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79976


Reviewed-by: Stefan B�rk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan B�rk <stefan@buerk.tech>