Make author avatar optional

_includes/author-profile.html

@@ -4,6 +4,7 @@
 
 <div itemscope itemtype="http://schema.org/Person">
 
+  {% if author.avatar %}
     <div class="author__avatar">
       {% if author.avatar contains "://" %}
       	<img src="{{ author.avatar }}" alt="{{ author.name }}">
@@ -11,6 +12,7 @@
       	<img src="{{ author.avatar | prepend: "/" | absolute_url }}" class="author__avatar" alt="{{ author.name }}">
       {% endif %}
     </div>
+  {% endif %}
 
   <div class="author__content">
     <h3 class="author__name">{{ author.name }}</h3>