fix: the isAlgoliaSearch variable set to boolean type (#519)

* fix: the isAlgoliaSearch set to boolean type

* fix: use searchBox when disabled algoliaSearchBox

Author: veaba

src/.vuepress/theme/components/Navbar.vue

@@ -26,7 +26,7 @@
       <SearchBox
         v-if="
           isAlgoliaSearch === false &&
-            !(
+            (
               $site.themeConfig.search !== false &&
               $page.frontmatter.search !== false
             )
@@ -68,7 +68,7 @@ export default {
     },
 
     isAlgoliaSearch() {
-      return this.algolia && this.algolia.apiKey && this.algolia.indexName
+      return !!(this.algolia && this.algolia.apiKey && this.algolia.indexName)
     }
   },