fix: dont change title when value is undefined (fix #396)

src/client/updaters/title.js

@@ -3,6 +3,10 @@
  *
  * @param  {String} title - the new title of the document
  */
-export default function updateTitle(title = document.title) {
+export default function updateTitle(title) {
+  if (title === undefined) {
+    return
+  }
+
   document.title = title
 }

src/shared/constants.js

@@ -4,7 +4,7 @@
 
 // set some sane defaults
 export const defaultInfo = {
-  title: '',
+  title: undefined,
   titleChunk: '',
   titleTemplate: '%s',
   htmlAttrs: {},