Get rid of 'undefined' in header and footers

Author: marcbachmann

lib/scripts/pdf_a4_portrait.js

@@ -106,7 +106,7 @@ function getContent (page) {
 
     var styles = document.querySelectorAll('link,style')
     styles = Array.prototype.reduce.call(styles, function (string, node) {
-      return string + node.outerHTML
+      return string + (node.outerHTML || '')
     }, '')
 
     // Wildcard headers e.g. <div id="pageHeader-first"> or <div id="pageHeader-0">