Do not set null as directory in finder
merged branch Danez/patch-2 (PR #220)

This PR was squashed before being merged into the master branch (closes #220).

Discussion
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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.

Commits
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dd7432d Do not set null as directory in finder