[BUGFIX] LocalDriver can be instantiated without basePath

The Local driver of FAL does not check if its configuration contains a
base path, leading to some hard-to-track errors when no path is given.

Resolves: #40505
Releases: 6.0
Change-Id: Id90a005f6cc9c167e3ab8df06a896cd078867e0a
Reviewed-on: http://review.typo3.org/14258
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter