[BUGFIX] Prevent TypeError in file related webhook messages

Since FileInterface->getPublicUrl() might return
`NULL`, corresponding constructor arguments are
updated to now expect `?string`.

Resolves: #103051
Releases: main, 12.4
Change-Id: I8d16c6b9361694733dc2c5c4b2e69693fe25cff4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82979


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>