Prevent the site test from failing when the avatar is not assigned (close #418)

Some rookies like to do this :(

_includes/sidebar.html

@@ -6,6 +6,7 @@
   <div class="profile-wrapper text-center">
     <div id="avatar">
       <a href="{{ '/' | relative_url }}" alt="avatar" class="mx-auto">
+        {% if site.avatar != '' and site.avatar %}
           {% capture avatar_url %}
             {%- if site.avatar contains '://' -%}
               {{ site.avatar }}
@@ -16,6 +17,7 @@
             {%- endif -%}
           {% endcapture %}
           <img src="{{ avatar_url }}" alt="avatar" onerror="this.style.display='none'">
+        {% endif %}
       </a>
     </div>