Crashes on empty CSS file

[torrottum]

cssshrink, both cli and lib crashes if the CSS input is empty

$ touch emptyfile.css

$ cssshrink emptyfile.css

/usr/local/lib/node_modules/cssshrink/lib/shrink.js:51
  var len = JSON.stringify(ast).length;
                               ^
TypeError: Cannot read property 'length' of undefined
    at traverseAST (/usr/local/lib/node_modules/cssshrink/lib/shrink.js:51:32)
    at Object.shrink (/usr/local/lib/node_modules/cssshrink/lib/shrink.js:66:9)
    at Object.<anonymous> (/usr/local/lib/node_modules/cssshrink/bin/cli.js:8:23)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3