Added null checker

Author: Micah Bule

lib/pdf.js

@@ -121,7 +121,7 @@ PDF.prototype.exec = function PdfExec (callback) {
     // Ignore if code has a value of 0 since that means PhantomJS has executed and exited successfully.
     // Also, as per your script and standards, having a code value of 1 means one can always assume that
     // an error occured.
-    if (typeof code !== 'undefined' && code !== 0) {
+    if ((typeof code !== 'undefined' && code !== null) && code !== 0) {
 
       var error = null;