Do not set null as directory in finder

With PR #218 I introduced a bug. (More like "made visible")
If Config::setDir() is not called (for example in .php_cs), the Config class calls Finder::setDir(null) in Config::getFinder().

This then leads to Symfony/Finder to throw "Directory name must not be empty."

These are some tests for config and finder. One of them fails.
Fix follows in some minutes.