[BUGFIX] Fatal error on creating new frontend user

The File service has a negated check for empty filenames.

Resolves: #60153
Change-Id: Ib4e80d89345ff1682afc5b9ff4d05d225c375e5e
Reviewed-on: https://review.typo3.org/31445
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer