Resolving constant paths and nil values.

When resolving a constant path that uses a variable with no value (e.g. the
locals created by block arguments) the ConstantPath class would raise an error.
This commit changes ConstantPath#resolve to return earlier when dealing with
nil values, preventing said error.

This fixes #114.
Merge branch 'master' of github.com:YorickPeterse/ruby-lint