Fixed handling of zero-byte-files

When linting a zero-byte-files, lesslint throws an error, when calling toCSS on a non-object.

PR includes fix & test
Instead of creating a new `describe` section, can you add this test to the LessFile section above.

Since you have a different `file`, you can create it in your test  by just moving that 

    file = new LessFile(filePath, {}, grunt)

into your `it`.