Fix: search button is invisible when the menu is empty (iissnan#42)

Author: iamKyun

layout/_partials/header/menu.njk

@@ -1,6 +1,6 @@
 {% import 'menu-item.njk' as menu_item with context %}
 
-{%- if theme.menu %}
+{%- if theme.menu or theme.algolia_search.enable or theme.local_search.enable %}
 <nav class="site-nav">
   <ul id="menu" class="main-menu menu">
     {%- for name, path in theme.menu %}