Ignore empty files (close #66)

Author: mishanga

lib/csscomb.js

@@ -90,11 +90,9 @@ Comb.prototype = {
      * @param {String} filename
      */
     processString: function(text, filename) {
+        if (!text) return text;
         var tree;
         var string = JSON.stringify;
-        if (typeof text === 'undefined') {
-            throw new Error('Undefined file content ' + filename + ': ' + string(text));
-        }
         try {
             tree = gonzales.srcToCSSP(text);
         } catch (e) {