Source maps should have a 'names' property

Fix for #1009. The `names` property of source maps isn't optional as per the [source map v3 spec](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit). It can however just be an empty array, which is what this commit adds.

The [mozilla/source-map](https://github.com/mozilla/source-map) library had to special case for Sass, but other implementations might not do this.