Fix: hide archives in sidebar if equal to 0.

If in main config of hexo `archive_dir` is set to '/' then archives will not generate by hexo-generator-archive: https://github.com/hexojs/hexo-generator-archive/blob/master/lib/generator.js#L19

Idea take from #639.

layout/_macro/sidebar.swig

@@ -32,12 +32,15 @@
           <p class="site-description motion-element" itemprop="description">{{ theme.description }}</p>
         </div>
         <nav class="site-state motion-element">
-          <div class="site-state-item site-state-posts">
-            <a href="{{ url_for(theme.menu.archives) }}">
-              <span class="site-state-item-count">{{ site.posts.length }}</span>
-              <span class="site-state-item-name">{{ __('state.posts') }}</span>
-            </a>
-          </div>
+
+          {% if config.archive_dir != '/' %}
+            <div class="site-state-item site-state-posts">
+              <a href="{{ url_for(theme.menu.archives) }}">
+                <span class="site-state-item-count">{{ site.posts.length }}</span>
+                <span class="site-state-item-name">{{ __('state.posts') }}</span>
+              </a>
+            </div>
+          {% endif %}
 
           {% if site.categories.length > 0 %}
             <div class="site-state-item site-state-categories">