fix: allows no rel attribute on external links in the nav (#2338)

* fix: allows no rel attribute on external links in the nav

* fix: documents the falsy rel option

* fix: clarifies documentation

* fix: explicitly uses  for rel attr

* fix: removes unnecessary condition
Co-authored-by: NBillyyyyy3320 <newsbielt703@gmail.com>
Co-authored-by: NBillyyyyy3320 <newsbielt703@gmail.com>

packages/@vuepress/theme-default/components/NavLink.vue

@@ -71,10 +71,13 @@ export default {
       if (this.isNonHttpURI) {
         return null
       }
+      if (this.item.rel === false) {
+        return null
+      }
       if (this.item.rel) {
         return this.item.rel
       }
-      return this.isBlankTarget ? 'noopener noreferrer' : ''
+      return this.isBlankTarget ? 'noopener noreferrer' : null
     }
   },
 

packages/docs/docs/theme/default-theme-config.md

@@ -69,14 +69,14 @@ module.exports = {
 }
 ```
 
-Outbound links automatically get `target="_blank" rel="noopener noreferrer"`. You can offer `target` and `rel` to customize the attributes:
+Outbound links automatically get `target="_blank" rel="noopener noreferrer"`. You can offer `target` and `rel` to customize the attributes. Setting `rel: false` as will disable the `rel` attribute for a link:
 
 ``` js
 // .vuepress/config.js
 module.exports = {
   themeConfig: {
     nav: [
-      { text: 'External', link: 'https://google.com', target:'_self', rel:'' },
+      { text: 'External', link: 'https://google.com', target:'_self', rel:false },
       { text: 'Guide', link: '/guide/', target:'_blank' }
     ]
   }
@@ -355,7 +355,7 @@ You can improve the search result by [setting `tags` in frontmatter](../guide/fr
 
 ```yaml
 ---
-tags: 
+tags:
   - configuration
   - theme
   - indexing