[BUGFIX] Avoid exception when trying to download a non-existing file

This patch fixes a regression introduced by change
https://review.typo3.org/c/Packages/TYPO3.CMS/+/65126 which lead to
an HTTP 500 exception instead of returning HTTP 404 by making sure
that FileDumpController->isFileValid() is not called with NULL as
parameter.

Resolves: #92957
Related: #91754
Releases: master, 10.4, 9.5
Change-Id: I824606867063f421f1a6fdfde4788846ddcc41a7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66947


Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>