[BUGFIX] Em: Get preconfigured distribution causes warning

This patch adds an early return for empty file name which happens if an
extension isn't installed.

Resolves: #53641
Releases: 6.2
Change-Id: I1cc3605bf5cddfd4e83c363b974a83d20172d0ce
Reviewed-on: https://review.typo3.org/25401
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter