Handle File resources with no parameters

[sathieu]

Overview

This pull request fixes: NoMethodError: undefined method `key?' for nil:NilClass with this kind of declaration:

file { '/path/to/purge-this-dir/except-this-file':
}

Checklist

• Make sure that all of the tests pass, and fix any that don't. Just run rake in your checkout directory, or review the CI job triggered whenever you push to a pull request.
• Make sure that there is 100% test coverage by running rake coverage:spec or ignoring untestable sections of code with # :nocov comments. If you need help getting to 100% coverage please ask; however, don't just submit code with no tests.
• If you have added a new command line option, we would greatly appreciate a corresponding integration test that exercises it from start to finish. This is optional but recommended.
• If you have added any new gem dependencies, make sure those gems are licensed under the MIT or Apache 2.0 license. We cannot add any dependencies on gems licensed under GPL.
• If you have added any new gem dependencies, make sure you've checked in a copy of the .gem file into the vendor/cache directory.

[kpaulisse]

Many thanks @sathieu - I've confirmed the problem and that this is a good fix. Will merge shortly!