Fix for bug 420009

Fixed bug where if user deleted the resource location, Krita would no longer load, giving an error trying to create the brushes folder.

Now if resource location exists but is an empty string, it gets replaced with the default resource location.

Maybe this bug only affects Windows users... I'm not sure what happens if the resource location is empty string on Linux/Mac/Android, but on Windows it causes an unrecoverable error, and no version of Krita based on master will ever run again. If empty string works fine on other systems, this may need to be changed to check for OS before changing it to the default resource location. I'm only testing on Windows, and I couldn't do anything with master until I made this fix.