Exit 0 when analysisFiles is empty

[leonelgalan]

Engine shouldn't error when analysisFiles is empty. This is common when running codeclimate analyze in a single non "style" file. See #8 for details.

If you want to test this locally:

• Clone the right branch: https://github.com/leonelgalan/codeclimate-stylelint/tree/exit-0
• Get into the folder and run the tests: cd test && sh setup.sh && cd ..
• Build the image locally docker build -t codeclimate/codeclimate-stylelint .