Add return type declaration to FileDumpController::dumpAction
For historical reasons FileDumpController::dumpAction was allowed to return
null. It didn't specify a nullable return type declaration because it was
unclear whether v9 will require php 7.2 or not.
With https://review.typo3.org/55585 FileDumpController was reworked to
always return a PSR-7 Response but missed to update the function signature.
That should be adapted.