fix crash when loading an empty config file

YAML returns `false` when the file contains no real data, even if it does contain comments. This breaks Rubocop, which is expecting a `Hash`.

This is a real-world issue because `rubocop-todo.yml` instructs the user to remove items one by one until everything's green, but doing so causes a crash when the final item is removed.

This pull request simply ensures that a `Hash` is used if YAML returns `false`.