Don't throw error on empty SCSS files

[DanielGibbsNZ]

I'm using this plugin to dynamically extract SCSS files and sometimes one of the files I need to extract variables from doesn't have any variables in it (or is totally empty). This causes the error Error while extracting declaration to be thrown.

Would it be possible to somehow allow it to continue even if there are no variables in one of the files?

[EnoahNetzach]

Would be possible for you to add at least an e2e test that fails in this case in a PR?

[DanielGibbsNZ]

See #3. I've fixed the issue and also added an empty stylesheet to the e2e tests which causes the tests to fail without my fix. I'm not sure if you want to fix it this way but at least will give you an idea of the issue.