AssertionError in empty files

Step by Step to error

echo > emptyfile.js
npm i minify
npx minify emptyfile.js > emptyfile.min.js

Could you please tell me what kind of an error do you have? And add test please, so this bug never returns again :)

I think rather than just taking "" as default. This case could be an Error("No content found in the file") This will make it easier to add file type detection (as per #70), as a blank file can not be detected.

What do you think @coderaiser ?