Handle empty filename bug

Author: grevory

lib/DfpUtils.js

@@ -35,5 +35,9 @@ module.exports.assetByteArray = function (filename) {
   var fs = require('fs');
   var bitmap = fs.readFileSync(filename);
 
+  if (!filename) {
+    return '';
+  }
+
   return new Buffer(bitmap).toString('base64');
 };