[BUGFIX] if basePath is not defined, assume empty string

[mkarulin]

Some drivers like fal_s3 don't define basePath in the flexform (They require processConfigt to be called). I was unable to get this working by patching fal_s3. Still, if the array key "basePath" is not declared, it shoud be concidered to be an empty string and not cause the website to crash.