fix crash when loading an empty config file

[sinisterchipmunk]

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.

[coveralls]

Coverage decreased (-0.09%) when pulling 71437be on sinisterchipmunk:support-empty-config-files into 415e648 on bbatsov:master.

[jonas054]

👍 Great! Please add a line about the bug fix in CHANGELOG.md.

[coveralls]

Coverage decreased (-0.09%) when pulling 7912e68 on sinisterchipmunk:support-empty-config-files into 415e648 on bbatsov:master.

[jonas054]

Good, but it looks like you also need to rebase on top of the latest master. While you're at it, you can squash your two commits into one.

[bbatsov]

I've squashed and rebased this myself. Thanks!